diff --git a/libc/hdr/types/CMakeLists.txt b/libc/hdr/types/CMakeLists.txt
index 89414f419a7f0..1ef1a2bee6f80 100644
--- a/libc/hdr/types/CMakeLists.txt
+++ b/libc/hdr/types/CMakeLists.txt
@@ -135,6 +135,14 @@ add_proxy_header_library(
     libc.include.llvm-libc-types.struct_tm
 )
 
+add_proxy_header_library(
+  rsize_t
+  HDRS
+    rsize_t.h
+  FULL_BUILD_DEPENDS
+    libc.include.llvm-libc-types.rsize_t
+)
+
 add_proxy_header_library(
   size_t
   HDRS
diff --git a/libc/hdr/types/rsize_t.h b/libc/hdr/types/rsize_t.h
new file mode 100644
index 0000000000000..130e6c308e264
--- /dev/null
+++ b/libc/hdr/types/rsize_t.h
@@ -0,0 +1,23 @@
+//===-- Proxy for rsize_t -------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_HDR_TYPES_SIZE_T_H
+#define LLVM_LIBC_HDR_TYPES_SIZE_T_H
+
+#ifdef LIBC_FULL_BUILD
+
+#include "include/llvm-libc-types/rsize_t.h"
+
+#else
+
+#define __need_rsize_t
+#include <stddef.h>
+#undef __need_rsize_t
+
+#endif // LIBC_FULL_BUILD
+
+#endif // LLVM_LIBC_HDR_TYPES_SIZE_T_H
diff --git a/libc/include/llvm-libc-types/CMakeLists.txt b/libc/include/llvm-libc-types/CMakeLists.txt
index bbeebbed2790a..d72de9fd72732 100644
--- a/libc/include/llvm-libc-types/CMakeLists.txt
+++ b/libc/include/llvm-libc-types/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_header(off64_t HDR off64_t.h)
+add_header(rsize_t HDR rsize_t.h)
 add_header(size_t HDR size_t.h)
 add_header(ssize_t HDR ssize_t.h)
 add_header(__atfork_callback_t HDR __atfork_callback_t.h)
diff --git a/libc/include/llvm-libc-types/rsize_t.h b/libc/include/llvm-libc-types/rsize_t.h
new file mode 100644
index 0000000000000..dc8be02bee592
--- /dev/null
+++ b/libc/include/llvm-libc-types/rsize_t.h
@@ -0,0 +1,18 @@
+//===-- Definition of size_t types ----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_TYPES_RSIZE_T_H
+#define LLVM_LIBC_TYPES_RSIZE_T_H
+
+#ifdef LIBC_HAS_ANNEX_K
+
+typedef __SIZE_TYPE__ rsize_t;
+
+#endif // LIBC_HAS_ANNEX_K
+
+#endif // LLVM_LIBC_TYPES_RSIZE_T_H