diff --git a/BUILD b/BUILD
index 00bd8fd5fb..b2ac9bbdec 100644
--- a/BUILD
+++ b/BUILD
@@ -106,6 +106,7 @@ cc_library(
         "upb/arena.c",
         "upb/decode.c",
         "upb/encode.c",
+        "upb/internal/arena.h",
         "upb/internal/decode.h",
         "upb/internal/table.h",
         "upb/internal/upb.h",
@@ -269,8 +270,8 @@ cc_library(
         "upb/decode.h",
         "upb/decode_fast.c",
         "upb/decode_fast.h",
+        "upb/internal/arena.h",
         "upb/internal/decode.h",
-        "upb/internal/upb.h",
         "upb/msg.h",
         "upb/msg_internal.h",
     ],
@@ -406,6 +407,7 @@ cc_library(
 cc_library(
     name = "json",
     srcs = [
+        "upb/internal/arena.h",
         "upb/internal/upb.h",
         "upb/json_decode.c",
         "upb/json_encode.c",
diff --git a/upb/arena.c b/upb/arena.c
index 6cee9cfcb2..85eaf78c02 100644
--- a/upb/arena.c
+++ b/upb/arena.c
@@ -28,7 +28,7 @@
 #include "upb/arena.h"
 
 #include "upb/alloc.h"
-#include "upb/internal/upb.h"
+#include "upb/internal/arena.h"
 
 // Must be last.
 #include "upb/port_def.inc"
diff --git a/upb/decode.h b/upb/decode.h
index 0c09497feb..985e3e4c39 100644
--- a/upb/decode.h
+++ b/upb/decode.h
@@ -32,6 +32,7 @@
 #ifndef UPB_DECODE_H_
 #define UPB_DECODE_H_
 
+#include "upb/arena.h"
 #include "upb/extension_registry.h"
 #include "upb/msg.h"
 
diff --git a/upb/internal/arena.h b/upb/internal/arena.h
new file mode 100644
index 0000000000..76707a30e0
--- /dev/null
+++ b/upb/internal/arena.h
@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2009-2021, Google LLC
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *     * Neither the name of Google LLC nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL Google LLC BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef UPB_INTERNAL_ARENA_H_
+#define UPB_INTERNAL_ARENA_H_
+
+#include "upb/arena.h"
+
+// Must be last.
+#include "upb/port_def.inc"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct mem_block mem_block;
+
+struct upb_Arena {
+  _upb_ArenaHead head;
+  /* Stores cleanup metadata for this arena.
+   * - a pointer to the current cleanup counter.
+   * - a boolean indicating if there is an unowned initial block.  */
+  uintptr_t cleanup_metadata;
+
+  /* Allocator to allocate arena blocks.  We are responsible for freeing these
+   * when we are destroyed. */
+  upb_alloc* block_alloc;
+  uint32_t last_size;
+
+  /* When multiple arenas are fused together, each arena points to a parent
+   * arena (root points to itself). The root tracks how many live arenas
+   * reference it. */
+  uint32_t refcount; /* Only used when a->parent == a */
+  struct upb_Arena* parent;
+
+  /* Linked list of blocks to free/cleanup. */
+  mem_block *freelist, *freelist_tail;
+};
+
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#include "upb/port_undef.inc"
+
+#endif /* UPB_INTERNAL_ARENA_H_ */
+
diff --git a/upb/internal/decode.h b/upb/internal/decode.h
index 708874c9c7..d1bc30a4c2 100644
--- a/upb/internal/decode.h
+++ b/upb/internal/decode.h
@@ -36,7 +36,7 @@
 #include <setjmp.h>
 
 #include "upb/decode.h"
-#include "upb/internal/upb.h"
+#include "upb/internal/arena.h"
 #include "upb/msg_internal.h"
 #include "third_party/utf8_range/utf8_range.h"
 
diff --git a/upb/internal/upb.h b/upb/internal/upb.h
index cdc8bfc228..7f72710020 100644
--- a/upb/internal/upb.h
+++ b/upb/internal/upb.h
@@ -30,30 +30,12 @@
 
 #include "upb/upb.h"
 
-struct mem_block;
-typedef struct mem_block mem_block;
+// Must be last.
+#include "upb/port_def.inc"
 
-struct upb_Arena {
-  _upb_ArenaHead head;
-  /* Stores cleanup metadata for this arena.
-   * - a pointer to the current cleanup counter.
-   * - a boolean indicating if there is an unowned initial block.  */
-  uintptr_t cleanup_metadata;
-
-  /* Allocator to allocate arena blocks.  We are responsible for freeing these
-   * when we are destroyed. */
-  upb_alloc* block_alloc;
-  uint32_t last_size;
-
-  /* When multiple arenas are fused together, each arena points to a parent
-   * arena (root points to itself). The root tracks how many live arenas
-   * reference it. */
-  uint32_t refcount; /* Only used when a->parent == a */
-  struct upb_Arena* parent;
-
-  /* Linked list of blocks to free/cleanup. */
-  mem_block *freelist, *freelist_tail;
-};
+#ifdef __cplusplus
+extern "C" {
+#endif
 
 // Encodes a float or double that is round-trippable, but as short as possible.
 // These routines are not fully optimal (not guaranteed to be shortest), but are
@@ -65,4 +47,10 @@ enum { kUpb_RoundTripBufferSize = 32 };
 void _upb_EncodeRoundTripDouble(double val, char* buf, size_t size);
 void _upb_EncodeRoundTripFloat(float val, char* buf, size_t size);
 
+#ifdef __cplusplus
+} /* extern "C" */
+#endif
+
+#include "upb/port_undef.inc"
+
 #endif /* UPB_INTERNAL_UPB_H_ */