diff --git a/src/google/protobuf/bridge/BUILD.bazel b/src/google/protobuf/bridge/BUILD.bazel
new file mode 100644
index 000000000000..b76bbae8f647
--- /dev/null
+++ b/src/google/protobuf/bridge/BUILD.bazel
@@ -0,0 +1,11 @@
+load("@rules_proto//proto:defs.bzl", "proto_library")
+
+package(
+    default_visibility = ["//visibility:public"],
+)
+
+proto_library(
+    name = "message_set_proto",
+    srcs = ["message_set.proto"],
+    strip_import_prefix = "/src",
+)
\ No newline at end of file
diff --git a/src/google/protobuf/bridge/message_set.proto b/src/google/protobuf/bridge/message_set.proto
new file mode 100644
index 000000000000..2f23f9b71ae2
--- /dev/null
+++ b/src/google/protobuf/bridge/message_set.proto
@@ -0,0 +1,76 @@
+// Protocol Buffers - Google's data interchange format
+// Copyright 2007 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// 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 Inc. 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 THE COPYRIGHT
+// OWNER OR CONTRIBUTORS 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.
+
+// Author: kenton@google.com (Kenton Varda)
+//
+// This is proto2's version of MessageSet.  See go/messageset to learn what
+// MessageSets are and how they are used.
+//
+// In proto2, we implement MessageSet in terms of extensions, except with a
+// special wire format for backwards-compatibility.  To define a message that
+// goes in a MessageSet in proto2, you must declare within that message's
+// scope an extension of MessageSet named "message_set_extension" and with
+// the field number matching the type ID.  So, for example, this proto1 code:
+//   message Foo {
+//     enum TypeId { MESSAGE_TYPE_ID = 1234; }
+//   }
+// becomes this proto2 code:
+//   message Foo {
+//     extend google.protobuf.bridge.MessageSet {
+//       optional Foo message_set_extension = 1234;
+//     }
+//   }
+//
+// Now you can use the usual proto2 extensions accessors to access this
+// message.  For example, the proto1 code:
+//   MessageSet mset;
+//   Foo* foo = mset.get_mutable<Foo>();
+// becomes this proto2 code:
+//   google::protobuf::bridge::MessageSet mset;
+//   Foo* foo = mset.MutableExtension(Foo::message_set_extension);
+//
+// Of course, new code that doesn't have backwards-compatibility requirements
+// should just use extensions themselves and not worry about MessageSet.
+
+syntax = "proto2";
+
+package google.protobuf.bridge;
+
+option java_outer_classname = "MessageSetProtos";
+option java_multiple_files = true;
+option cc_enable_arenas = true;
+option objc_class_prefix = "GPB";
+
+// This is proto2's version of MessageSet.
+message MessageSet {
+  option message_set_wire_format = true;
+
+  extensions 4 to max;
+}