diff --git a/upb/upb.int.h b/upb/upb.int.h
new file mode 100644
index 0000000000..b857560e4e
--- /dev/null
+++ b/upb/upb.int.h
@@ -0,0 +1,29 @@
+
+#ifndef UPB_INT_H_
+#define UPB_INT_H_
+
+#include "upb/upb.h"
+
+struct mem_block;
+typedef struct mem_block mem_block;
+
+struct upb_arena {
+  _upb_arena_head head;
+  uint32_t *cleanups;
+
+  /* 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;
+};
+
+#endif  /* UPB_INT_H_ */