diff --git a/tests/pb/test_decoder.cc b/tests/pb/test_decoder.cc
index 49544b2fa2..e44f60c461 100644
--- a/tests/pb/test_decoder.cc
+++ b/tests/pb/test_decoder.cc
@@ -1098,7 +1098,7 @@ void test_valid() {
       LINE("%u:{")
       LINE("  <")
       LINE("  %u:(5)\"abcde")
-      LINE("  %u:\"")
+      LINE("    %u:\"")
       LINE("  >")
       LINE("}")
       LINE(">"), msg_fn, UPB_DESCRIPTOR_TYPE_STRING,
diff --git a/tools/dump_cinit.lua b/tools/dump_cinit.lua
index 34d9dd1ec6..34d6ac6297 100644
--- a/tools/dump_cinit.lua
+++ b/tools/dump_cinit.lua
@@ -351,6 +351,7 @@ end
 local function well_known_type(m)
   local type_map = {}
   type_map["google.protobuf.Any"]         = "UPB_WELLKNOWN_ANY"
+  type_map["google.protobuf.FieldMask"]   = "UPB_WELLKNOWN_FIELDMASK"
   type_map["google.protobuf.Duration"]    = "UPB_WELLKNOWN_DURATION"
   type_map["google.protobuf.Timestamp"]   = "UPB_WELLKNOWN_TIMESTAMP"
   type_map["google.protobuf.Value"]       = "UPB_WELLKNOWN_VALUE"
diff --git a/upb/def.c b/upb/def.c
index 379be0beb0..223fb1d6b6 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -373,6 +373,8 @@ static void assign_msg_wellknowntype(upb_msgdef *m) {
   }
   if (!strcmp(name, "google.protobuf.Any")) {
     m->well_known_type = UPB_WELLKNOWN_ANY;
+  } else if (!strcmp(name, "google.protobuf.FieldMask")) {
+    m->well_known_type = UPB_WELLKNOWN_FIELDMASK;
   } else if (!strcmp(name, "google.protobuf.Duration")) {
     m->well_known_type = UPB_WELLKNOWN_DURATION;
   } else if (!strcmp(name, "google.protobuf.Timestamp")) {
diff --git a/upb/def.h b/upb/def.h
index 98b458ab0c..19b2a20b5d 100644
--- a/upb/def.h
+++ b/upb/def.h
@@ -286,6 +286,7 @@ typedef enum {
 typedef enum {
   UPB_WELLKNOWN_UNSPECIFIED,
   UPB_WELLKNOWN_ANY,
+  UPB_WELLKNOWN_FIELDMASK,
   UPB_WELLKNOWN_DURATION,
   UPB_WELLKNOWN_TIMESTAMP,
   /* number wrappers */
diff --git a/upb/json/parser.c b/upb/json/parser.c
index 08056d39af..42539ee327 100644
--- a/upb/json/parser.c
+++ b/upb/json/parser.c
@@ -63,6 +63,12 @@ static bool is_string_wrapper_object(upb_json_parser *p);
 static bool does_string_wrapper_start(upb_json_parser *p);
 static bool does_string_wrapper_end(upb_json_parser *p);
 
+static bool is_fieldmask_object(upb_json_parser *p);
+static bool does_fieldmask_start(upb_json_parser *p);
+static bool does_fieldmask_end(upb_json_parser *p);
+static void start_fieldmask_object(upb_json_parser *p);
+static void end_fieldmask_object(upb_json_parser *p);
+
 static void start_wrapper_object(upb_json_parser *p);
 static void end_wrapper_object(upb_json_parser *p);
 
@@ -1164,6 +1170,9 @@ static bool start_stringval(upb_json_parser *p) {
   if (is_top_level(p)) {
     if (is_string_wrapper_object(p)) {
       start_wrapper_object(p);
+    } else if (is_wellknown_msg(p, UPB_WELLKNOWN_FIELDMASK)) {
+      start_fieldmask_object(p);
+      return true;
     } else if (is_wellknown_msg(p, UPB_WELLKNOWN_TIMESTAMP) ||
                is_wellknown_msg(p, UPB_WELLKNOWN_DURATION)) {
       start_object(p);
@@ -1177,6 +1186,12 @@ static bool start_stringval(upb_json_parser *p) {
       return false;
     }
     start_wrapper_object(p);
+  } else if (does_fieldmask_start(p)) {
+    if (!start_subobject(p)) {
+      return false;
+    }
+    start_fieldmask_object(p);
+    return true;
   } else if (is_wellknown_field(p, UPB_WELLKNOWN_TIMESTAMP) ||
              is_wellknown_field(p, UPB_WELLKNOWN_DURATION)) {
     if (!start_subobject(p)) {
@@ -1320,8 +1335,8 @@ static bool end_stringval_nontop(upb_json_parser *p) {
 
     case UPB_TYPE_STRING: {
       upb_selector_t sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
-      p->top--;
       upb_sink_endstr(&p->top->sink, sel);
+      p->top--;
       break;
     }
 
@@ -1370,6 +1385,16 @@ static bool end_stringval_nontop(upb_json_parser *p) {
 }
 
 static bool end_stringval(upb_json_parser *p) {
+  /* FieldMask's stringvals have been ended when handling them. Only need to
+   * close FieldMask here.*/
+  if (does_fieldmask_end(p)) {
+    end_fieldmask_object(p);
+    if (!is_top_level(p)) {
+      end_subobject(p);
+    }
+    return true;
+  }
+
   if (!end_stringval_nontop(p)) {
     return false;
   }
@@ -1391,7 +1416,8 @@ static bool end_stringval(upb_json_parser *p) {
   }
 
   if (is_wellknown_msg(p, UPB_WELLKNOWN_TIMESTAMP) ||
-      is_wellknown_msg(p, UPB_WELLKNOWN_DURATION)) {
+      is_wellknown_msg(p, UPB_WELLKNOWN_DURATION) ||
+      is_wellknown_msg(p, UPB_WELLKNOWN_FIELDMASK)) {
     end_object(p);
     if (!is_top_level(p)) {
       end_subobject(p);
@@ -1654,6 +1680,75 @@ static bool end_timestamp_zone(upb_json_parser *p, const char *ptr) {
   return true;
 }
 
+static void start_fieldmask_path_text(upb_json_parser *p, const char *ptr) {
+  capture_begin(p, ptr);
+}
+
+static bool end_fieldmask_path_text(upb_json_parser *p, const char *ptr) {
+  if (!capture_end(p, ptr)) {
+    return false;
+  }
+}
+
+static bool start_fieldmask_path(upb_json_parser *p) {
+  upb_jsonparser_frame *inner;
+  upb_selector_t sel;
+
+  if (!check_stack(p)) return false;
+
+  /* Start a new parser frame: parser frames correspond one-to-one with
+   * handler frames, and string events occur in a sub-frame. */
+  inner = p->top + 1;
+  sel = getsel_for_handlertype(p, UPB_HANDLER_STARTSTR);
+  upb_sink_startstr(&p->top->sink, sel, 0, &inner->sink);
+  inner->m = p->top->m;
+  inner->f = p->top->f;
+  inner->name_table = NULL;
+  inner->is_map = false;
+  inner->is_mapentry = false;
+  inner->is_any = false;
+  inner->any_frame = NULL;
+  inner->is_unknown_field = false;
+  p->top = inner;
+
+  multipart_startaccum(p);
+  return true;
+}
+
+static bool lower_camel_push(
+    upb_json_parser *p, upb_selector_t sel, const char *ptr, size_t len) {
+  const char *limit = ptr + len;
+  bool first = true;
+  for (;ptr < limit; ptr++) {
+    if (*ptr >= 'A' && *ptr <= 'Z' && !first) {
+      char lower = tolower(*ptr);
+      upb_sink_putstring(&p->top->sink, sel, "_", 1, NULL);
+      upb_sink_putstring(&p->top->sink, sel, &lower, 1, NULL);
+    } else {
+      upb_sink_putstring(&p->top->sink, sel, ptr, 1, NULL);
+    }
+    first = false;
+  }
+  return true;
+}
+
+static bool end_fieldmask_path(upb_json_parser *p) {
+  upb_selector_t sel;
+
+  if (!lower_camel_push(
+           p, getsel_for_handlertype(p, UPB_HANDLER_STRING),
+           p->accumulated, p->accumulated_len)) {
+    return false;
+  }
+
+  sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
+  upb_sink_endstr(&p->top->sink, sel);
+  p->top--;
+
+  multipart_end(p);
+  return true;
+}
+
 static void start_member(upb_json_parser *p) {
   UPB_ASSERT(!p->top->f);
   multipart_startaccum(p);
@@ -1713,7 +1808,7 @@ static bool parse_mapentry_key(upb_json_parser *p) {
       sel = getsel_for_handlertype(p, UPB_HANDLER_STRING);
       upb_sink_putstring(&subsink, sel, buf, len, NULL);
       sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
-      upb_sink_endstr(&p->top->sink, sel);
+      upb_sink_endstr(&subsink, sel);
       multipart_end(p);
       break;
     }
@@ -2255,6 +2350,31 @@ static bool is_string_wrapper(const upb_msgdef *m) {
          type == UPB_WELLKNOWN_BYTESVALUE;
 }
 
+static bool is_fieldmask(const upb_msgdef *m) {
+  upb_wellknowntype_t type = upb_msgdef_wellknowntype(m);
+  return type == UPB_WELLKNOWN_FIELDMASK;
+}
+
+static void start_fieldmask_object(upb_json_parser *p) {
+  const char *membername = "paths";
+
+  start_object(p);
+
+  /* Set up context for parsing value */
+  start_member(p);
+  capture_begin(p, membername);
+  capture_end(p, membername + 5);
+  end_membername(p);
+
+  start_array(p);
+}
+
+static void end_fieldmask_object(upb_json_parser *p) {
+  end_array(p);
+  end_member(p);
+  end_object(p);
+}
+
 static void start_wrapper_object(upb_json_parser *p) {
   const char *membername = "value";
 
@@ -2393,6 +2513,20 @@ static bool is_string_wrapper_object(upb_json_parser *p) {
   return p->top->m != NULL && is_string_wrapper(p->top->m);
 }
 
+static bool does_fieldmask_start(upb_json_parser *p) {
+  return p->top->f != NULL &&
+         upb_fielddef_issubmsg(p->top->f) &&
+         is_fieldmask(upb_fielddef_msgsubdef(p->top->f));
+}
+
+static bool does_fieldmask_end(upb_json_parser *p) {
+  return p->top->m != NULL && is_fieldmask(p->top->m);
+}
+
+static bool is_fieldmask_object(upb_json_parser *p) {
+  return p->top->m != NULL && is_fieldmask(p->top->m);
+}
+
 #define CHECK_RETURN_TOP(x) if (!(x)) goto error
 
 
@@ -2414,26 +2548,27 @@ static bool is_string_wrapper_object(upb_json_parser *p) {
  * final state once, when the closing '"' is seen. */
 
 
-#line 2576 "upb/json/parser.rl"
+#line 2730 "upb/json/parser.rl"
 
 
 
-#line 2422 "upb/json/parser.c"
+#line 2556 "upb/json/parser.c"
 static const char _json_actions[] = {
 	0, 1, 0, 1, 1, 1, 3, 1, 
 	4, 1, 6, 1, 7, 1, 8, 1, 
 	9, 1, 10, 1, 11, 1, 12, 1, 
-	13, 1, 21, 1, 23, 1, 24, 1, 
-	26, 1, 27, 1, 28, 1, 30, 1, 
-	32, 1, 33, 1, 34, 1, 35, 1, 
-	37, 1, 38, 2, 4, 9, 2, 5, 
-	6, 2, 7, 3, 2, 7, 9, 2, 
-	14, 15, 2, 16, 17, 2, 18, 19, 
-	2, 22, 20, 2, 24, 26, 2, 29, 
-	2, 2, 30, 38, 2, 31, 20, 2, 
-	33, 38, 2, 34, 38, 2, 35, 38, 
-	2, 36, 25, 2, 37, 38, 4, 14, 
-	15, 16, 17
+	13, 1, 24, 1, 26, 1, 28, 1, 
+	29, 1, 31, 1, 32, 1, 33, 1, 
+	35, 1, 37, 1, 38, 1, 39, 1, 
+	40, 1, 42, 1, 43, 2, 4, 9, 
+	2, 5, 6, 2, 7, 3, 2, 7, 
+	9, 2, 14, 15, 2, 16, 17, 2, 
+	18, 19, 2, 21, 23, 2, 22, 20, 
+	2, 27, 25, 2, 29, 31, 2, 34, 
+	2, 2, 35, 43, 2, 36, 25, 2, 
+	38, 43, 2, 39, 43, 2, 40, 43, 
+	2, 41, 30, 2, 42, 43, 3, 21, 
+	23, 24, 4, 14, 15, 16, 17
 };
 
 static const short _json_key_offsets[] = {
@@ -2446,12 +2581,12 @@ static const short _json_key_offsets[] = {
 	156, 158, 160, 162, 163, 165, 167, 168, 
 	170, 172, 173, 175, 177, 178, 180, 182, 
 	183, 185, 187, 191, 193, 195, 196, 197, 
-	198, 199, 201, 206, 215, 216, 216, 216, 
-	221, 226, 231, 232, 233, 234, 235, 235, 
-	236, 237, 238, 238, 239, 240, 241, 241, 
-	246, 247, 251, 256, 261, 266, 270, 270, 
-	273, 276, 279, 282, 285, 288, 288, 288, 
-	288, 288
+	198, 199, 201, 206, 208, 210, 212, 221, 
+	222, 222, 222, 227, 232, 237, 238, 239, 
+	240, 241, 241, 242, 243, 244, 244, 245, 
+	246, 247, 247, 252, 253, 257, 262, 267, 
+	272, 276, 276, 279, 282, 285, 288, 291, 
+	294, 294, 294, 294, 294, 294
 };
 
 static const char _json_trans_keys[] = {
@@ -2480,18 +2615,18 @@ static const char _json_trans_keys[] = {
 	57, 58, 48, 57, 48, 57, 58, 48, 
 	57, 48, 57, 43, 45, 46, 90, 48, 
 	57, 48, 57, 58, 48, 48, 34, 48, 
-	57, 43, 45, 90, 48, 57, 34, 45, 
-	91, 102, 110, 116, 123, 48, 57, 34, 
-	32, 93, 125, 9, 13, 32, 44, 93, 
-	9, 13, 32, 93, 125, 9, 13, 97, 
-	108, 115, 101, 117, 108, 108, 114, 117, 
-	101, 32, 34, 125, 9, 13, 34, 32, 
-	58, 9, 13, 32, 93, 125, 9, 13, 
-	32, 44, 125, 9, 13, 32, 44, 125, 
-	9, 13, 32, 34, 9, 13, 32, 9, 
-	13, 32, 9, 13, 32, 9, 13, 32, 
+	57, 43, 45, 90, 48, 57, 34, 44, 
+	34, 44, 34, 44, 34, 45, 91, 102, 
+	110, 116, 123, 48, 57, 34, 32, 93, 
+	125, 9, 13, 32, 44, 93, 9, 13, 
+	32, 93, 125, 9, 13, 97, 108, 115, 
+	101, 117, 108, 108, 114, 117, 101, 32, 
+	34, 125, 9, 13, 34, 32, 58, 9, 
+	13, 32, 93, 125, 9, 13, 32, 44, 
+	125, 9, 13, 32, 44, 125, 9, 13, 
+	32, 34, 9, 13, 32, 9, 13, 32, 
 	9, 13, 32, 9, 13, 32, 9, 13, 
-	0
+	32, 9, 13, 32, 9, 13, 0
 };
 
 static const char _json_single_lengths[] = {
@@ -2504,12 +2639,12 @@ static const char _json_single_lengths[] = {
 	0, 0, 0, 1, 0, 0, 1, 0, 
 	0, 1, 0, 0, 1, 0, 0, 1, 
 	0, 0, 4, 0, 0, 1, 1, 1, 
-	1, 0, 3, 7, 1, 0, 0, 3, 
-	3, 3, 1, 1, 1, 1, 0, 1, 
-	1, 1, 0, 1, 1, 1, 0, 3, 
-	1, 2, 3, 3, 3, 2, 0, 1, 
-	1, 1, 1, 1, 1, 0, 0, 0, 
-	0, 0
+	1, 0, 3, 2, 2, 2, 7, 1, 
+	0, 0, 3, 3, 3, 1, 1, 1, 
+	1, 0, 1, 1, 1, 0, 1, 1, 
+	1, 0, 3, 1, 2, 3, 3, 3, 
+	2, 0, 1, 1, 1, 1, 1, 1, 
+	0, 0, 0, 0, 0, 0
 };
 
 static const char _json_range_lengths[] = {
@@ -2522,12 +2657,12 @@ static const char _json_range_lengths[] = {
 	1, 1, 1, 0, 1, 1, 0, 1, 
 	1, 0, 1, 1, 0, 1, 1, 0, 
 	1, 1, 0, 1, 1, 0, 0, 0, 
-	0, 1, 1, 1, 0, 0, 0, 1, 
-	1, 1, 0, 0, 0, 0, 0, 0, 
-	0, 0, 0, 0, 0, 0, 0, 1, 
-	0, 1, 1, 1, 1, 1, 0, 1, 
-	1, 1, 1, 1, 1, 0, 0, 0, 
-	0, 0
+	0, 1, 1, 0, 0, 0, 1, 0, 
+	0, 0, 1, 1, 1, 0, 0, 0, 
+	0, 0, 0, 0, 0, 0, 0, 0, 
+	0, 0, 1, 0, 1, 1, 1, 1, 
+	1, 0, 1, 1, 1, 1, 1, 1, 
+	0, 0, 0, 0, 0, 0
 };
 
 static const short _json_index_offsets[] = {
@@ -2540,12 +2675,12 @@ static const short _json_index_offsets[] = {
 	166, 168, 170, 172, 174, 176, 178, 180, 
 	182, 184, 186, 188, 190, 192, 194, 196, 
 	198, 200, 202, 207, 209, 211, 213, 215, 
-	217, 219, 221, 226, 235, 237, 238, 239, 
-	244, 249, 254, 256, 258, 260, 262, 263, 
-	265, 267, 269, 270, 272, 274, 276, 277, 
-	282, 284, 288, 293, 298, 303, 307, 308, 
-	311, 314, 317, 320, 323, 326, 327, 328, 
-	329, 330
+	217, 219, 221, 226, 229, 232, 235, 244, 
+	246, 247, 248, 253, 258, 263, 265, 267, 
+	269, 271, 272, 274, 276, 278, 279, 281, 
+	283, 285, 286, 291, 293, 297, 302, 307, 
+	312, 316, 317, 320, 323, 326, 329, 332, 
+	335, 336, 337, 338, 339, 340
 };
 
 static const unsigned char _json_indicies[] = {
@@ -2577,64 +2712,67 @@ static const unsigned char _json_indicies[] = {
 	88, 1, 89, 89, 90, 91, 1, 92, 
 	1, 93, 1, 94, 1, 95, 1, 96, 
 	1, 97, 1, 98, 1, 99, 99, 100, 
-	98, 1, 101, 102, 103, 104, 105, 106, 
-	107, 102, 1, 108, 1, 109, 110, 112, 
-	113, 1, 112, 111, 114, 115, 113, 114, 
-	1, 115, 1, 1, 115, 111, 116, 1, 
-	117, 1, 118, 1, 119, 1, 120, 121, 
-	1, 122, 1, 123, 1, 124, 125, 1, 
-	126, 1, 127, 1, 128, 129, 130, 131, 
-	129, 1, 132, 1, 133, 134, 133, 1, 
-	134, 1, 1, 134, 135, 136, 137, 138, 
-	136, 1, 139, 140, 131, 139, 1, 140, 
-	130, 140, 1, 141, 142, 142, 1, 143, 
-	143, 1, 144, 144, 1, 145, 145, 1, 
-	146, 146, 1, 147, 147, 1, 1, 1, 
-	1, 1, 1, 0
+	98, 1, 102, 1, 101, 104, 105, 103, 
+	1, 1, 101, 106, 107, 108, 109, 110, 
+	111, 112, 107, 1, 113, 1, 114, 115, 
+	117, 118, 1, 117, 116, 119, 120, 118, 
+	119, 1, 120, 1, 1, 120, 116, 121, 
+	1, 122, 1, 123, 1, 124, 1, 125, 
+	126, 1, 127, 1, 128, 1, 129, 130, 
+	1, 131, 1, 132, 1, 133, 134, 135, 
+	136, 134, 1, 137, 1, 138, 139, 138, 
+	1, 139, 1, 1, 139, 140, 141, 142, 
+	143, 141, 1, 144, 145, 136, 144, 1, 
+	145, 135, 145, 1, 146, 147, 147, 1, 
+	148, 148, 1, 149, 149, 1, 150, 150, 
+	1, 151, 151, 1, 152, 152, 1, 1, 
+	1, 1, 1, 1, 1, 0
 };
 
 static const char _json_trans_targs[] = {
-	1, 0, 2, 104, 3, 6, 10, 13, 
-	16, 103, 4, 3, 103, 4, 5, 7, 
-	8, 9, 105, 11, 12, 106, 14, 15, 
-	107, 16, 17, 108, 18, 18, 19, 20, 
-	21, 22, 108, 21, 22, 24, 25, 31, 
-	109, 26, 28, 27, 29, 30, 33, 110, 
-	34, 33, 110, 34, 32, 35, 36, 37, 
-	38, 39, 33, 110, 34, 41, 42, 46, 
-	42, 46, 43, 45, 44, 111, 48, 49, 
+	1, 0, 2, 107, 3, 6, 10, 13, 
+	16, 106, 4, 3, 106, 4, 5, 7, 
+	8, 9, 108, 11, 12, 109, 14, 15, 
+	110, 16, 17, 111, 18, 18, 19, 20, 
+	21, 22, 111, 21, 22, 24, 25, 31, 
+	112, 26, 28, 27, 29, 30, 33, 113, 
+	34, 33, 113, 34, 32, 35, 36, 37, 
+	38, 39, 33, 113, 34, 41, 42, 46, 
+	42, 46, 43, 45, 44, 114, 48, 49, 
 	50, 51, 52, 53, 54, 55, 56, 57, 
 	58, 59, 60, 61, 62, 63, 64, 65, 
 	66, 67, 73, 72, 68, 69, 70, 71, 
-	72, 112, 74, 67, 72, 76, 78, 79, 
-	82, 87, 91, 95, 77, 113, 113, 80, 
-	79, 77, 80, 81, 83, 84, 85, 86, 
-	113, 88, 89, 90, 113, 92, 93, 94, 
-	113, 95, 96, 102, 97, 97, 98, 99, 
-	100, 101, 102, 100, 101, 113, 103, 103, 
-	103, 103, 103, 103
+	72, 115, 74, 67, 72, 76, 116, 76, 
+	116, 77, 79, 81, 82, 85, 90, 94, 
+	98, 80, 117, 117, 83, 82, 80, 83, 
+	84, 86, 87, 88, 89, 117, 91, 92, 
+	93, 117, 95, 96, 97, 117, 98, 99, 
+	105, 100, 100, 101, 102, 103, 104, 105, 
+	103, 104, 117, 106, 106, 106, 106, 106, 
+	106
 };
 
 static const char _json_trans_actions[] = {
-	0, 0, 84, 78, 33, 0, 0, 0, 
-	96, 39, 25, 0, 35, 0, 0, 0, 
+	0, 0, 92, 86, 35, 0, 0, 0, 
+	104, 41, 27, 0, 37, 0, 0, 0, 
 	0, 0, 0, 0, 0, 0, 0, 0, 
-	0, 0, 72, 31, 27, 0, 0, 25, 
-	29, 29, 75, 0, 0, 0, 0, 0, 
+	0, 0, 80, 33, 29, 0, 0, 27, 
+	31, 31, 83, 0, 0, 0, 0, 0, 
 	3, 0, 0, 0, 0, 0, 5, 15, 
-	0, 0, 51, 7, 13, 0, 54, 9, 
-	9, 9, 57, 60, 11, 17, 17, 17, 
+	0, 0, 53, 7, 13, 0, 56, 9, 
+	9, 9, 59, 62, 11, 17, 17, 17, 
 	0, 0, 0, 19, 0, 21, 23, 0, 
 	0, 0, 0, 0, 0, 0, 0, 0, 
 	0, 0, 0, 0, 0, 0, 0, 0, 
-	0, 102, 63, 102, 0, 0, 0, 0, 
-	0, 69, 0, 66, 66, 84, 78, 33, 
-	0, 0, 0, 96, 39, 49, 81, 25, 
-	0, 35, 0, 0, 0, 0, 0, 0, 
-	90, 0, 0, 0, 93, 0, 0, 0, 
-	87, 0, 72, 31, 27, 0, 0, 25, 
-	29, 29, 75, 0, 0, 99, 0, 37, 
-	43, 45, 41, 47
+	0, 114, 65, 114, 0, 0, 0, 0, 
+	0, 71, 0, 68, 68, 77, 25, 0, 
+	110, 74, 92, 86, 35, 0, 0, 0, 
+	104, 41, 51, 89, 27, 0, 37, 0, 
+	0, 0, 0, 0, 0, 98, 0, 0, 
+	0, 101, 0, 0, 0, 95, 0, 80, 
+	33, 29, 0, 0, 27, 31, 31, 83, 
+	0, 0, 107, 0, 39, 45, 47, 43, 
+	49
 };
 
 static const char _json_eof_actions[] = {
@@ -2651,8 +2789,8 @@ static const char _json_eof_actions[] = {
 	0, 0, 0, 0, 0, 0, 0, 0, 
 	0, 0, 0, 0, 0, 0, 0, 0, 
 	0, 0, 0, 0, 0, 0, 0, 0, 
-	37, 43, 45, 41, 47, 0, 0, 0, 
-	0, 0
+	0, 0, 0, 39, 45, 47, 43, 49, 
+	0, 0, 0, 0, 0, 0
 };
 
 static const int json_start = 1;
@@ -2661,11 +2799,12 @@ static const int json_en_number_machine = 23;
 static const int json_en_string_machine = 32;
 static const int json_en_duration_machine = 40;
 static const int json_en_timestamp_machine = 47;
-static const int json_en_value_machine = 75;
+static const int json_en_fieldmask_machine = 75;
+static const int json_en_value_machine = 78;
 static const int json_en_main = 1;
 
 
-#line 2579 "upb/json/parser.rl"
+#line 2733 "upb/json/parser.rl"
 
 size_t parse(void *closure, const void *hd, const char *buf, size_t size,
              const upb_bufhandle *handle) {
@@ -2688,7 +2827,7 @@ size_t parse(void *closure, const void *hd, const char *buf, size_t size,
   capture_resume(parser, buf);
 
   
-#line 2692 "upb/json/parser.c"
+#line 2831 "upb/json/parser.c"
 	{
 	int _klen;
 	unsigned int _trans;
@@ -2763,99 +2902,121 @@ _match:
 		switch ( *_acts++ )
 		{
 	case 1:
-#line 2427 "upb/json/parser.rl"
+#line 2561 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; goto _again;} }
 	break;
 	case 2:
-#line 2429 "upb/json/parser.rl"
+#line 2563 "upb/json/parser.rl"
 	{ p--; {stack[top++] = cs; cs = 23;goto _again;} }
 	break;
 	case 3:
-#line 2433 "upb/json/parser.rl"
+#line 2567 "upb/json/parser.rl"
 	{ start_text(parser, p); }
 	break;
 	case 4:
-#line 2434 "upb/json/parser.rl"
+#line 2568 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_text(parser, p)); }
 	break;
 	case 5:
-#line 2440 "upb/json/parser.rl"
+#line 2574 "upb/json/parser.rl"
 	{ start_hex(parser); }
 	break;
 	case 6:
-#line 2441 "upb/json/parser.rl"
+#line 2575 "upb/json/parser.rl"
 	{ hexdigit(parser, p); }
 	break;
 	case 7:
-#line 2442 "upb/json/parser.rl"
+#line 2576 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_hex(parser)); }
 	break;
 	case 8:
-#line 2448 "upb/json/parser.rl"
+#line 2582 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(escape(parser, p)); }
 	break;
 	case 9:
-#line 2454 "upb/json/parser.rl"
+#line 2588 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; goto _again;} }
 	break;
 	case 10:
-#line 2466 "upb/json/parser.rl"
+#line 2600 "upb/json/parser.rl"
 	{ start_duration_base(parser, p); }
 	break;
 	case 11:
-#line 2467 "upb/json/parser.rl"
+#line 2601 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_duration_base(parser, p)); }
 	break;
 	case 12:
-#line 2469 "upb/json/parser.rl"
+#line 2603 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; goto _again;} }
 	break;
 	case 13:
-#line 2474 "upb/json/parser.rl"
+#line 2608 "upb/json/parser.rl"
 	{ start_timestamp_base(parser, p); }
 	break;
 	case 14:
-#line 2475 "upb/json/parser.rl"
+#line 2609 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_timestamp_base(parser, p)); }
 	break;
 	case 15:
-#line 2477 "upb/json/parser.rl"
+#line 2611 "upb/json/parser.rl"
 	{ start_timestamp_fraction(parser, p); }
 	break;
 	case 16:
-#line 2478 "upb/json/parser.rl"
+#line 2612 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_timestamp_fraction(parser, p)); }
 	break;
 	case 17:
-#line 2480 "upb/json/parser.rl"
+#line 2614 "upb/json/parser.rl"
 	{ start_timestamp_zone(parser, p); }
 	break;
 	case 18:
-#line 2481 "upb/json/parser.rl"
+#line 2615 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_timestamp_zone(parser, p)); }
 	break;
 	case 19:
-#line 2483 "upb/json/parser.rl"
+#line 2617 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; goto _again;} }
 	break;
 	case 20:
-#line 2488 "upb/json/parser.rl"
+#line 2622 "upb/json/parser.rl"
+	{ start_fieldmask_path_text(parser, p); }
+	break;
+	case 21:
+#line 2623 "upb/json/parser.rl"
+	{ end_fieldmask_path_text(parser, p); }
+	break;
+	case 22:
+#line 2628 "upb/json/parser.rl"
+	{ start_fieldmask_path(parser); }
+	break;
+	case 23:
+#line 2629 "upb/json/parser.rl"
+	{ end_fieldmask_path(parser); }
+	break;
+	case 24:
+#line 2635 "upb/json/parser.rl"
+	{ p--; {cs = stack[--top]; goto _again;} }
+	break;
+	case 25:
+#line 2640 "upb/json/parser.rl"
 	{
         if (is_wellknown_msg(parser, UPB_WELLKNOWN_TIMESTAMP)) {
           {stack[top++] = cs; cs = 47;goto _again;}
         } else if (is_wellknown_msg(parser, UPB_WELLKNOWN_DURATION)) {
           {stack[top++] = cs; cs = 40;goto _again;}
+        } else if (is_wellknown_msg(parser, UPB_WELLKNOWN_FIELDMASK)) {
+          {stack[top++] = cs; cs = 75;goto _again;}
         } else {
           {stack[top++] = cs; cs = 32;goto _again;}
         }
       }
 	break;
-	case 21:
-#line 2499 "upb/json/parser.rl"
-	{ p--; {stack[top++] = cs; cs = 75;goto _again;} }
+	case 26:
+#line 2653 "upb/json/parser.rl"
+	{ p--; {stack[top++] = cs; cs = 78;goto _again;} }
 	break;
-	case 22:
-#line 2504 "upb/json/parser.rl"
+	case 27:
+#line 2658 "upb/json/parser.rl"
 	{
         if (is_wellknown_msg(parser, UPB_WELLKNOWN_ANY)) {
           start_any_member(parser, p);
@@ -2864,12 +3025,12 @@ _match:
         }
       }
 	break;
-	case 23:
-#line 2511 "upb/json/parser.rl"
+	case 28:
+#line 2665 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_membername(parser)); }
 	break;
-	case 24:
-#line 2514 "upb/json/parser.rl"
+	case 29:
+#line 2668 "upb/json/parser.rl"
 	{
         if (is_wellknown_msg(parser, UPB_WELLKNOWN_ANY)) {
           end_any_member(parser, p);
@@ -2878,8 +3039,8 @@ _match:
         }
       }
 	break;
-	case 25:
-#line 2525 "upb/json/parser.rl"
+	case 30:
+#line 2679 "upb/json/parser.rl"
 	{
         if (is_wellknown_msg(parser, UPB_WELLKNOWN_ANY)) {
           start_any_object(parser, p);
@@ -2888,8 +3049,8 @@ _match:
         }
       }
 	break;
-	case 26:
-#line 2534 "upb/json/parser.rl"
+	case 31:
+#line 2688 "upb/json/parser.rl"
 	{
         if (is_wellknown_msg(parser, UPB_WELLKNOWN_ANY)) {
           CHECK_RETURN_TOP(end_any_object(parser, p));
@@ -2898,55 +3059,55 @@ _match:
         }
       }
 	break;
-	case 27:
-#line 2546 "upb/json/parser.rl"
+	case 32:
+#line 2700 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(start_array(parser)); }
 	break;
-	case 28:
-#line 2550 "upb/json/parser.rl"
+	case 33:
+#line 2704 "upb/json/parser.rl"
 	{ end_array(parser); }
 	break;
-	case 29:
-#line 2555 "upb/json/parser.rl"
+	case 34:
+#line 2709 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(start_number(parser, p)); }
 	break;
-	case 30:
-#line 2556 "upb/json/parser.rl"
+	case 35:
+#line 2710 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_number(parser, p)); }
 	break;
-	case 31:
-#line 2558 "upb/json/parser.rl"
+	case 36:
+#line 2712 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(start_stringval(parser)); }
 	break;
-	case 32:
-#line 2559 "upb/json/parser.rl"
+	case 37:
+#line 2713 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_stringval(parser)); }
 	break;
-	case 33:
-#line 2561 "upb/json/parser.rl"
+	case 38:
+#line 2715 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_bool(parser, true)); }
 	break;
-	case 34:
-#line 2563 "upb/json/parser.rl"
+	case 39:
+#line 2717 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_bool(parser, false)); }
 	break;
-	case 35:
-#line 2565 "upb/json/parser.rl"
+	case 40:
+#line 2719 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_null(parser)); }
 	break;
-	case 36:
-#line 2567 "upb/json/parser.rl"
+	case 41:
+#line 2721 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(start_subobject_full(parser)); }
 	break;
-	case 37:
-#line 2568 "upb/json/parser.rl"
+	case 42:
+#line 2722 "upb/json/parser.rl"
 	{ end_subobject_full(parser); }
 	break;
-	case 38:
-#line 2573 "upb/json/parser.rl"
+	case 43:
+#line 2727 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; goto _again;} }
 	break;
-#line 2950 "upb/json/parser.c"
+#line 3111 "upb/json/parser.c"
 		}
 	}
 
@@ -2963,32 +3124,32 @@ _again:
 	while ( __nacts-- > 0 ) {
 		switch ( *__acts++ ) {
 	case 0:
-#line 2425 "upb/json/parser.rl"
+#line 2559 "upb/json/parser.rl"
 	{ p--; {cs = stack[--top]; 	if ( p == pe )
 		goto _test_eof;
 goto _again;} }
 	break;
-	case 30:
-#line 2556 "upb/json/parser.rl"
+	case 35:
+#line 2710 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_number(parser, p)); }
 	break;
-	case 33:
-#line 2561 "upb/json/parser.rl"
+	case 38:
+#line 2715 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_bool(parser, true)); }
 	break;
-	case 34:
-#line 2563 "upb/json/parser.rl"
+	case 39:
+#line 2717 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_bool(parser, false)); }
 	break;
-	case 35:
-#line 2565 "upb/json/parser.rl"
+	case 40:
+#line 2719 "upb/json/parser.rl"
 	{ CHECK_RETURN_TOP(end_null(parser)); }
 	break;
-	case 37:
-#line 2568 "upb/json/parser.rl"
+	case 42:
+#line 2722 "upb/json/parser.rl"
 	{ end_subobject_full(parser); }
 	break;
-#line 2992 "upb/json/parser.c"
+#line 3153 "upb/json/parser.c"
 		}
 	}
 	}
@@ -2996,7 +3157,7 @@ goto _again;} }
 	_out: {}
 	}
 
-#line 2601 "upb/json/parser.rl"
+#line 2755 "upb/json/parser.rl"
 
   if (p != pe) {
     upb_status_seterrf(&parser->status, "Parse error at '%.*s'\n", pe - p, p);
@@ -3019,6 +3180,7 @@ static bool end(void *closure, const void *hd) {
   /* Prevent compile warning on unused static constants. */
   UPB_UNUSED(json_start);
   UPB_UNUSED(json_en_duration_machine);
+  UPB_UNUSED(json_en_fieldmask_machine);
   UPB_UNUSED(json_en_number_machine);
   UPB_UNUSED(json_en_string_machine);
   UPB_UNUSED(json_en_timestamp_machine);
@@ -3027,7 +3189,7 @@ static bool end(void *closure, const void *hd) {
 
   parse(parser, hd, &eof_ch, 0, NULL);
 
-  return parser->current_state >= 103;
+  return parser->current_state >= 106;
 }
 
 static void json_parser_reset(upb_json_parser *p) {
@@ -3044,13 +3206,13 @@ static void json_parser_reset(upb_json_parser *p) {
 
   /* Emit Ragel initialization of the parser. */
   
-#line 3048 "upb/json/parser.c"
+#line 3210 "upb/json/parser.c"
 	{
 	cs = json_start;
 	top = 0;
 	}
 
-#line 2648 "upb/json/parser.rl"
+#line 2803 "upb/json/parser.rl"
   p->current_state = cs;
   p->parser_top = top;
   accumulate_clear(p);
diff --git a/upb/json/parser.rl b/upb/json/parser.rl
index e5845153a8..70a45cb6c8 100644
--- a/upb/json/parser.rl
+++ b/upb/json/parser.rl
@@ -61,6 +61,12 @@ static bool is_string_wrapper_object(upb_json_parser *p);
 static bool does_string_wrapper_start(upb_json_parser *p);
 static bool does_string_wrapper_end(upb_json_parser *p);
 
+static bool is_fieldmask_object(upb_json_parser *p);
+static bool does_fieldmask_start(upb_json_parser *p);
+static bool does_fieldmask_end(upb_json_parser *p);
+static void start_fieldmask_object(upb_json_parser *p);
+static void end_fieldmask_object(upb_json_parser *p);
+
 static void start_wrapper_object(upb_json_parser *p);
 static void end_wrapper_object(upb_json_parser *p);
 
@@ -1162,6 +1168,9 @@ static bool start_stringval(upb_json_parser *p) {
   if (is_top_level(p)) {
     if (is_string_wrapper_object(p)) {
       start_wrapper_object(p);
+    } else if (is_wellknown_msg(p, UPB_WELLKNOWN_FIELDMASK)) {
+      start_fieldmask_object(p);
+      return true;
     } else if (is_wellknown_msg(p, UPB_WELLKNOWN_TIMESTAMP) ||
                is_wellknown_msg(p, UPB_WELLKNOWN_DURATION)) {
       start_object(p);
@@ -1175,6 +1184,12 @@ static bool start_stringval(upb_json_parser *p) {
       return false;
     }
     start_wrapper_object(p);
+  } else if (does_fieldmask_start(p)) {
+    if (!start_subobject(p)) {
+      return false;
+    }
+    start_fieldmask_object(p);
+    return true;
   } else if (is_wellknown_field(p, UPB_WELLKNOWN_TIMESTAMP) ||
              is_wellknown_field(p, UPB_WELLKNOWN_DURATION)) {
     if (!start_subobject(p)) {
@@ -1318,8 +1333,8 @@ static bool end_stringval_nontop(upb_json_parser *p) {
 
     case UPB_TYPE_STRING: {
       upb_selector_t sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
-      p->top--;
       upb_sink_endstr(&p->top->sink, sel);
+      p->top--;
       break;
     }
 
@@ -1368,6 +1383,16 @@ static bool end_stringval_nontop(upb_json_parser *p) {
 }
 
 static bool end_stringval(upb_json_parser *p) {
+  /* FieldMask's stringvals have been ended when handling them. Only need to
+   * close FieldMask here.*/
+  if (does_fieldmask_end(p)) {
+    end_fieldmask_object(p);
+    if (!is_top_level(p)) {
+      end_subobject(p);
+    }
+    return true;
+  }
+
   if (!end_stringval_nontop(p)) {
     return false;
   }
@@ -1389,7 +1414,8 @@ static bool end_stringval(upb_json_parser *p) {
   }
 
   if (is_wellknown_msg(p, UPB_WELLKNOWN_TIMESTAMP) ||
-      is_wellknown_msg(p, UPB_WELLKNOWN_DURATION)) {
+      is_wellknown_msg(p, UPB_WELLKNOWN_DURATION) ||
+      is_wellknown_msg(p, UPB_WELLKNOWN_FIELDMASK)) {
     end_object(p);
     if (!is_top_level(p)) {
       end_subobject(p);
@@ -1652,6 +1678,75 @@ static bool end_timestamp_zone(upb_json_parser *p, const char *ptr) {
   return true;
 }
 
+static void start_fieldmask_path_text(upb_json_parser *p, const char *ptr) {
+  capture_begin(p, ptr);
+}
+
+static bool end_fieldmask_path_text(upb_json_parser *p, const char *ptr) {
+  if (!capture_end(p, ptr)) {
+    return false;
+  }
+}
+
+static bool start_fieldmask_path(upb_json_parser *p) {
+  upb_jsonparser_frame *inner;
+  upb_selector_t sel;
+
+  if (!check_stack(p)) return false;
+
+  /* Start a new parser frame: parser frames correspond one-to-one with
+   * handler frames, and string events occur in a sub-frame. */
+  inner = p->top + 1;
+  sel = getsel_for_handlertype(p, UPB_HANDLER_STARTSTR);
+  upb_sink_startstr(&p->top->sink, sel, 0, &inner->sink);
+  inner->m = p->top->m;
+  inner->f = p->top->f;
+  inner->name_table = NULL;
+  inner->is_map = false;
+  inner->is_mapentry = false;
+  inner->is_any = false;
+  inner->any_frame = NULL;
+  inner->is_unknown_field = false;
+  p->top = inner;
+
+  multipart_startaccum(p);
+  return true;
+}
+
+static bool lower_camel_push(
+    upb_json_parser *p, upb_selector_t sel, const char *ptr, size_t len) {
+  const char *limit = ptr + len;
+  bool first = true;
+  for (;ptr < limit; ptr++) {
+    if (*ptr >= 'A' && *ptr <= 'Z' && !first) {
+      char lower = tolower(*ptr);
+      upb_sink_putstring(&p->top->sink, sel, "_", 1, NULL);
+      upb_sink_putstring(&p->top->sink, sel, &lower, 1, NULL);
+    } else {
+      upb_sink_putstring(&p->top->sink, sel, ptr, 1, NULL);
+    }
+    first = false;
+  }
+  return true;
+}
+
+static bool end_fieldmask_path(upb_json_parser *p) {
+  upb_selector_t sel;
+
+  if (!lower_camel_push(
+           p, getsel_for_handlertype(p, UPB_HANDLER_STRING),
+           p->accumulated, p->accumulated_len)) {
+    return false;
+  }
+
+  sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
+  upb_sink_endstr(&p->top->sink, sel);
+  p->top--;
+
+  multipart_end(p);
+  return true;
+}
+
 static void start_member(upb_json_parser *p) {
   UPB_ASSERT(!p->top->f);
   multipart_startaccum(p);
@@ -1711,7 +1806,7 @@ static bool parse_mapentry_key(upb_json_parser *p) {
       sel = getsel_for_handlertype(p, UPB_HANDLER_STRING);
       upb_sink_putstring(&subsink, sel, buf, len, NULL);
       sel = getsel_for_handlertype(p, UPB_HANDLER_ENDSTR);
-      upb_sink_endstr(&p->top->sink, sel);
+      upb_sink_endstr(&subsink, sel);
       multipart_end(p);
       break;
     }
@@ -2253,6 +2348,31 @@ static bool is_string_wrapper(const upb_msgdef *m) {
          type == UPB_WELLKNOWN_BYTESVALUE;
 }
 
+static bool is_fieldmask(const upb_msgdef *m) {
+  upb_wellknowntype_t type = upb_msgdef_wellknowntype(m);
+  return type == UPB_WELLKNOWN_FIELDMASK;
+}
+
+static void start_fieldmask_object(upb_json_parser *p) {
+  const char *membername = "paths";
+
+  start_object(p);
+
+  /* Set up context for parsing value */
+  start_member(p);
+  capture_begin(p, membername);
+  capture_end(p, membername + 5);
+  end_membername(p);
+
+  start_array(p);
+}
+
+static void end_fieldmask_object(upb_json_parser *p) {
+  end_array(p);
+  end_member(p);
+  end_object(p);
+}
+
 static void start_wrapper_object(upb_json_parser *p) {
   const char *membername = "value";
 
@@ -2391,6 +2511,20 @@ static bool is_string_wrapper_object(upb_json_parser *p) {
   return p->top->m != NULL && is_string_wrapper(p->top->m);
 }
 
+static bool does_fieldmask_start(upb_json_parser *p) {
+  return p->top->f != NULL &&
+         upb_fielddef_issubmsg(p->top->f) &&
+         is_fieldmask(upb_fielddef_msgsubdef(p->top->f));
+}
+
+static bool does_fieldmask_end(upb_json_parser *p) {
+  return p->top->m != NULL && is_fieldmask(p->top->m);
+}
+
+static bool is_fieldmask_object(upb_json_parser *p) {
+  return p->top->m != NULL && is_fieldmask(p->top->m);
+}
+
 #define CHECK_RETURN_TOP(x) if (!(x)) goto error
 
 
@@ -2483,6 +2617,24 @@ static bool is_string_wrapper_object(upb_json_parser *p) {
       @{ fhold; fret; }
     ;
 
+  fieldmask_path_text =
+    /[^",]/+
+      >{ start_fieldmask_path_text(parser, p); }
+      %{ end_fieldmask_path_text(parser, p); }
+    ;
+
+  fieldmask_path =
+    fieldmask_path_text
+      >{ start_fieldmask_path(parser); }
+      %{ end_fieldmask_path(parser); }
+    ;
+
+  fieldmask_machine :=
+    (fieldmask_path ("," fieldmask_path)*)?
+    '"'
+      @{ fhold; fret; }
+    ;
+
   string =
     '"'
       @{
@@ -2490,6 +2642,8 @@ static bool is_string_wrapper_object(upb_json_parser *p) {
           fcall timestamp_machine;
         } else if (is_wellknown_msg(parser, UPB_WELLKNOWN_DURATION)) {
           fcall duration_machine;
+        } else if (is_wellknown_msg(parser, UPB_WELLKNOWN_FIELDMASK)) {
+          fcall fieldmask_machine;
         } else {
           fcall string_machine;
         }
@@ -2620,6 +2774,7 @@ static bool end(void *closure, const void *hd) {
   /* Prevent compile warning on unused static constants. */
   UPB_UNUSED(json_start);
   UPB_UNUSED(json_en_duration_machine);
+  UPB_UNUSED(json_en_fieldmask_machine);
   UPB_UNUSED(json_en_number_machine);
   UPB_UNUSED(json_en_string_machine);
   UPB_UNUSED(json_en_timestamp_machine);
diff --git a/upb/json/printer.c b/upb/json/printer.c
index 5ed79a0f3d..2283d55423 100644
--- a/upb/json/printer.c
+++ b/upb/json/printer.c
@@ -747,6 +747,57 @@ static void *startseq_nokey(void *closure, const void *handler_data) {
   return closure;
 }
 
+static void *startseq_fieldmask(void *closure, const void *handler_data) {
+  upb_json_printer *p = closure;
+  UPB_UNUSED(handler_data);
+  p->depth_++;
+  p->first_elem_[p->depth_] = true;
+  print_data(p, "\"", 1);
+  return closure;
+}
+
+static bool endseq_fieldmask(void *closure, const void *handler_data) {
+  upb_json_printer *p = closure;
+  UPB_UNUSED(handler_data);
+  p->depth_--;
+  print_data(p, "\"", 1);
+  return true;
+}
+
+static void *repeated_startstr_fieldmask(
+    void *closure, const void *handler_data,
+    size_t size_hint) {
+  upb_json_printer *p = closure;
+  UPB_UNUSED(handler_data);
+  UPB_UNUSED(size_hint);
+  print_comma(p);
+  return p;
+}
+
+static size_t repeated_str_fieldmask(
+    void *closure, const void *handler_data,
+    const char *str, size_t len,
+    const upb_bufhandle *handle) {
+  const char* limit = str + len;
+  bool upper = false;
+  size_t result_len = 0;
+  for (; str < limit; str++) {
+    if (*str == '_') {
+      upper = true;
+      continue;
+    }
+    if (upper && *str >= 'a' && *str <= 'z') {
+      char upper_char = toupper(*str);
+      CHK(putstr(closure, handler_data, &upper_char, 1, handle));
+    } else {
+      CHK(putstr(closure, handler_data, str, 1, handle));
+    }
+    upper = false;
+    result_len++;
+  }
+  return result_len;
+}
+
 static void *startmap_nokey(void *closure, const void *handler_data) {
   upb_json_printer *p = closure;
   UPB_UNUSED(handler_data);
@@ -972,6 +1023,25 @@ void printer_sethandlers_any(const void *closure, upb_handlers *h) {
   UPB_UNUSED(closure);
 }
 
+/* Set up handlers for a fieldmask submessage. */
+void printer_sethandlers_fieldmask(const void *closure, upb_handlers *h) {
+  const upb_msgdef *md = upb_handlers_msgdef(h);
+  const upb_fielddef* f = upb_msgdef_itof(md, 1);
+
+  upb_handlerattr empty_attr = UPB_HANDLERATTR_INITIALIZER;
+
+  upb_handlers_setstartseq(h, f, startseq_fieldmask, &empty_attr);
+  upb_handlers_setendseq(h, f, endseq_fieldmask, &empty_attr);
+
+  upb_handlers_setstartmsg(h, printer_startmsg_noframe, &empty_attr);
+  upb_handlers_setendmsg(h, printer_endmsg_noframe, &empty_attr);
+
+  upb_handlers_setstartstr(h, f, repeated_startstr_fieldmask, &empty_attr);
+  upb_handlers_setstring(h, f, repeated_str_fieldmask, &empty_attr);
+
+  UPB_UNUSED(closure);
+}
+
 /* Set up handlers for a duration submessage. */
 void printer_sethandlers_duration(const void *closure, upb_handlers *h) {
   const upb_msgdef *md = upb_handlers_msgdef(h);
@@ -1128,6 +1198,9 @@ void printer_sethandlers(const void *closure, upb_handlers *h) {
     case UPB_WELLKNOWN_ANY:
       printer_sethandlers_any(closure, h);
       return;
+    case UPB_WELLKNOWN_FIELDMASK:
+      printer_sethandlers_fieldmask(closure, h);
+      return;
     case UPB_WELLKNOWN_DURATION:
       printer_sethandlers_duration(closure, h);
       return;
diff --git a/upb/pb/compile_decoder.c b/upb/pb/compile_decoder.c
index d147edf551..cec7b2a54e 100644
--- a/upb/pb/compile_decoder.c
+++ b/upb/pb/compile_decoder.c
@@ -666,8 +666,8 @@ static void generate_delimfield(compiler *c, const upb_fielddef *f,
     putop(c, OP_STARTSTR, getsel(f, UPB_HANDLER_STARTSTR));
     /* Need to emit even if no handler to skip past the string. */
     putop(c, OP_STRING, getsel(f, UPB_HANDLER_STRING));
-    putop(c, OP_POP);
     maybeput(c, OP_ENDSTR, h, f, UPB_HANDLER_ENDSTR);
+    putop(c, OP_POP);
     putop(c, OP_SETDELIM);
     putop(c, OP_CHECKDELIM, LABEL_LOOPBREAK);
     putchecktag(c, f, UPB_WIRE_TYPE_DELIMITED, LABEL_LOOPBREAK);
@@ -682,8 +682,8 @@ static void generate_delimfield(compiler *c, const upb_fielddef *f,
     putop(c, OP_PUSHLENDELIM);
     putop(c, OP_STARTSTR, getsel(f, UPB_HANDLER_STARTSTR));
     putop(c, OP_STRING, getsel(f, UPB_HANDLER_STRING));
-    putop(c, OP_POP);
     maybeput(c, OP_ENDSTR, h, f, UPB_HANDLER_ENDSTR);
+    putop(c, OP_POP);
     putop(c, OP_SETDELIM);
   }
 }