diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl.h b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
index 8243076e1c..e04988e708 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl.h
@@ -350,7 +350,7 @@ class ModuleRtpRtcpImpl : public RtpRtcp, public RTCPReceiver::ModuleRtpRtcp {
 
   // The processed RTT from RtcpRttStats.
   mutable Mutex mutex_rtt_;
-  int64_t rtt_ms_;
+  int64_t rtt_ms_ RTC_GUARDED_BY(mutex_rtt_);
 };
 
 }  // namespace webrtc
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl2.h b/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
index 512dfa08ce..1dde49d8b6 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
@@ -319,7 +319,7 @@ class ModuleRtpRtcpImpl2 final : public RtpRtcpInterface,
 
   // The processed RTT from RtcpRttStats.
   mutable Mutex mutex_rtt_;
-  int64_t rtt_ms_;
+  int64_t rtt_ms_ RTC_GUARDED_BY(mutex_rtt_);
 };
 
 }  // namespace webrtc