diff --git a/webrtc/build/common.gypi b/webrtc/build/common.gypi
index a77420c070..d5b79f9c13 100644
--- a/webrtc/build/common.gypi
+++ b/webrtc/build/common.gypi
@@ -142,11 +142,23 @@
     # support, |ffmpeg_branding| has to separately be set to a value that
     # includes H.264, for example "Chrome". If FFmpeg is built without H.264,
     # compilation succeeds but |H264DecoderImpl| fails to initialize.
+    # See also: |rtc_initialize_ffmpeg|.
     # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
     # http://www.openh264.org, https://www.ffmpeg.org/
     'rtc_use_h264%': 0,
 
     'conditions': [
+      # FFmpeg must be initialized for |H264DecoderImpl| to work. This can be
+      # done by WebRTC during |H264DecoderImpl::InitDecode| or externally.
+      # FFmpeg must only be initialized once. Projects that initialize FFmpeg
+      # externally, such as Chromium, must turn this flag off so that WebRTC
+      # does not also initialize.
+      ['build_with_chromium==0', {
+        'rtc_initialize_ffmpeg%': 1,
+      }, {
+        'rtc_initialize_ffmpeg%': 0,
+      }],
+
       ['build_with_chromium==1', {
         # Exclude pulse audio on Chromium since its prerequisites don't require
         # pulse audio.
diff --git a/webrtc/build/webrtc.gni b/webrtc/build/webrtc.gni
index f784ab1830..886b899543 100644
--- a/webrtc/build/webrtc.gni
+++ b/webrtc/build/webrtc.gni
@@ -96,9 +96,17 @@ declare_args() {
   # support, |ffmpeg_branding| has to separately be set to a value that
   # includes H.264, for example "Chrome". If FFmpeg is built without H.264,
   # compilation succeeds but |H264DecoderImpl| fails to initialize.
+  # See also: |rtc_initialize_ffmpeg|.
   # CHECK THE OPENH264, FFMPEG AND H.264 LICENSES/PATENTS BEFORE BUILDING.
   # http://www.openh264.org, https://www.ffmpeg.org/
   rtc_use_h264 = false
+
+  # FFmpeg must be initialized for |H264DecoderImpl| to work. This can be done
+  # by WebRTC during |H264DecoderImpl::InitDecode| or externally. FFmpeg must
+  # only be initialized once. Projects that initialize FFmpeg externally, such
+  # as Chromium, must turn this flag off so that WebRTC does not also
+  # initialize.
+  rtc_initialize_ffmpeg = !build_with_chromium
 }
 
 # A second declare_args block, so that declarations within it can
diff --git a/webrtc/modules/video_coding/BUILD.gn b/webrtc/modules/video_coding/BUILD.gn
index 115ed3588b..2d9e08d347 100644
--- a/webrtc/modules/video_coding/BUILD.gn
+++ b/webrtc/modules/video_coding/BUILD.gn
@@ -140,6 +140,9 @@ source_set("webrtc_h264") {
 
   if (rtc_use_h264) {
     defines += [ "WEBRTC_THIRD_PARTY_H264" ]
+    if (rtc_initialize_ffmpeg) {
+      defines += [ "WEBRTC_INITIALIZE_FFMPEG" ]
+    }
     sources += [
       "codecs/h264/h264_decoder_impl.cc",
       "codecs/h264/h264_decoder_impl.h",
diff --git a/webrtc/modules/video_coding/codecs/h264/h264.gypi b/webrtc/modules/video_coding/codecs/h264/h264.gypi
index 0e80561bdf..e82b0fd4cd 100644
--- a/webrtc/modules/video_coding/codecs/h264/h264.gypi
+++ b/webrtc/modules/video_coding/codecs/h264/h264.gypi
@@ -27,6 +27,13 @@
           'defines': [
             'WEBRTC_THIRD_PARTY_H264',
           ],
+          'conditions': [
+            ['rtc_initialize_ffmpeg==1', {
+              'defines': [
+                'WEBRTC_INITIALIZE_FFMPEG',
+              ],
+            }],
+          ],
           'dependencies': [
             '<(DEPTH)/third_party/ffmpeg/ffmpeg.gyp:ffmpeg',
             '<(DEPTH)/third_party/openh264/openh264.gyp:openh264_encoder',
diff --git a/webrtc/modules/video_coding/codecs/h264/h264_decoder_impl.cc b/webrtc/modules/video_coding/codecs/h264/h264_decoder_impl.cc
index 75d2bfa732..ea96f25a1e 100644
--- a/webrtc/modules/video_coding/codecs/h264/h264_decoder_impl.cc
+++ b/webrtc/modules/video_coding/codecs/h264/h264_decoder_impl.cc
@@ -34,8 +34,9 @@ const size_t kYPlaneIndex = 0;
 const size_t kUPlaneIndex = 1;
 const size_t kVPlaneIndex = 2;
 
-#if !defined(WEBRTC_CHROMIUM_BUILD)
+#if defined(WEBRTC_INITIALIZE_FFMPEG)
 
+rtc::CriticalSection ffmpeg_init_lock;
 bool ffmpeg_initialized = false;
 
 // Called by FFmpeg to do mutex operations if initialized using
@@ -61,10 +62,8 @@ int LockManagerOperation(void** lock, AVLockOp op)
   return -1;
 }
 
-// TODO(hbos): Assumed to be called on a single thread. Should DCHECK that
-// InitializeFFmpeg is only called on one thread or make it thread safe.
-// See https://bugs.chromium.org/p/webrtc/issues/detail?id=5427.
 void InitializeFFmpeg() {
+  rtc::CritScope cs(&ffmpeg_init_lock);
   if (!ffmpeg_initialized) {
     if (av_lockmgr_register(LockManagerOperation) < 0) {
       RTC_NOTREACHED() << "av_lockmgr_register failed.";
@@ -75,7 +74,7 @@ void InitializeFFmpeg() {
   }
 }
 
-#endif  // !defined(WEBRTC_CHROMIUM_BUILD)
+#endif  // defined(WEBRTC_INITIALIZE_FFMPEG)
 
 // Called by FFmpeg when it is done with a frame buffer, see AVGetBuffer2.
 void AVFreeBuffer2(void* opaque, uint8_t* data) {
@@ -179,13 +178,15 @@ int32_t H264DecoderImpl::InitDecode(const VideoCodec* codec_settings,
     return WEBRTC_VIDEO_CODEC_ERR_PARAMETER;
   }
 
-  // In Chromium FFmpeg will be initialized outside of WebRTC and we should not
-  // attempt to do so ourselves or it will be initialized twice.
-  // TODO(hbos): Put behind a different flag in case non-chromium project wants
-  // to initialize externally.
-  // See https://bugs.chromium.org/p/webrtc/issues/detail?id=5427.
-#if !defined(WEBRTC_CHROMIUM_BUILD)
-  // Make sure FFmpeg has been initialized.
+  // FFmpeg must have been initialized (with |av_lockmgr_register| and
+  // |av_register_all|) before we proceed. |InitializeFFmpeg| does this, which
+  // makes sense for WebRTC standalone. In other cases, such as Chromium, FFmpeg
+  // is initialized externally and calling |InitializeFFmpeg| would be
+  // thread-unsafe and result in FFmpeg being initialized twice, which could
+  // break other FFmpeg usage. See the |rtc_initialize_ffmpeg| flag.
+#if defined(WEBRTC_INITIALIZE_FFMPEG)
+  // Make sure FFmpeg has been initialized. Subsequent |InitializeFFmpeg| calls
+  // do nothing.
   InitializeFFmpeg();
 #endif