diff --git a/upb/def.c b/upb/def.c
index 1a83687788..c3c9d104fc 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -1331,7 +1331,7 @@ static bool create_fielddef(
           break;
         }
       }
-      assert(found);
+      UPB_ASSERT(found);
     }
   } else {
     /* extension field. */
diff --git a/upb/json/parser.rl b/upb/json/parser.rl
index 1180ce137f..eefdadf577 100644
--- a/upb/json/parser.rl
+++ b/upb/json/parser.rl
@@ -389,7 +389,7 @@ static upb_selector_t getsel_for_handlertype(upb_json_parser *p,
                                              upb_handlertype_t type) {
   upb_selector_t sel;
   bool ok = upb_handlers_getselector(p->top->f, type, &sel);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   return sel;
 }
 
@@ -414,7 +414,7 @@ static void set_name_table(upb_json_parser *p, upb_jsonparser_frame *frame) {
   const upb_json_parsermethod *method;
 
   ok = upb_inttable_lookupptr(&cache->methods, frame->m, &v);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   method = upb_value_getconstptr(v);
 
   frame->name_table = &method->name_table;
@@ -2019,7 +2019,7 @@ static void end_member(upb_json_parser *p) {
     /* send ENDSUBMSG in repeated-field-of-mapentries frame. */
     p->top--;
     ok = upb_handlers_getselector(mapfield, UPB_HANDLER_ENDSUBMSG, &sel);
-    UPB_ASSERT(ok);
+    UPB_ASSUME(ok);
     upb_sink_endsubmsg(p->top->sink, (p->top + 1)->sink, sel);
   }
 
diff --git a/upb/pb/compile_decoder.c b/upb/pb/compile_decoder.c
index f5c7d65646..44b310fe3f 100644
--- a/upb/pb/compile_decoder.c
+++ b/upb/pb/compile_decoder.c
@@ -910,10 +910,10 @@ const upb_pbdecodermethod *upb_pbcodecache_get(upb_pbcodecache *c,
   } else {
     g = mgroup_new(h, c->lazy);
     ok = upb_inttable_insertptr(&c->groups, md, upb_value_constptr(g));
-    UPB_ASSERT(ok);
+    UPB_ASSUME(ok);
   }
 
   ok = upb_inttable_lookupptr(&g->methods, h, &v);
-  UPB_ASSERT(ok);
+  UPB_ASSUME(ok);
   return upb_value_getptr(v);
 }
diff --git a/upb/port_def.inc b/upb/port_def.inc
index 992148e736..51f8eac96f 100644
--- a/upb/port_def.inc
+++ b/upb/port_def.inc
@@ -128,6 +128,18 @@ int msvc_vsnprintf(char* s, size_t n, const char* format, va_list arg);
 
 #define UPB_UNUSED(var) (void)var
 
+/* UPB_ASSUME(): in release mode, we tell the compiler to assume this is true.
+ */
+#ifdef NDEBUG
+#ifdef __GNUC__
+#define UPB_ASSUME(expr) if (!(expr)) __builtin_unreachable()
+#else
+#define UPB_ASSUME(expr) do {} if (false && (expr))
+#endif
+#else
+#define UPB_ASSUME(expr) assert(expr)
+#endif
+
 /* UPB_ASSERT(): in release mode, we use the expression without letting it be
  * evaluated.  This prevents "unused variable" warnings. */
 #ifdef NDEBUG
diff --git a/upb/port_undef.inc b/upb/port_undef.inc
index 6a4daa5076..e91e9644bb 100644
--- a/upb/port_undef.inc
+++ b/upb/port_undef.inc
@@ -12,6 +12,7 @@
 #undef UPB_MAX
 #undef UPB_MIN
 #undef UPB_UNUSED
+#undef UPB_ASSUME
 #undef UPB_ASSERT
 #undef UPB_ASSERT_DEBUGVAR
 #undef UPB_UNREACHABLE