diff --git a/webrtc/modules/congestion_controller/include/send_side_congestion_controller.h b/webrtc/modules/congestion_controller/include/send_side_congestion_controller.h
index 8130feb01e..58eba25a7f 100644
--- a/webrtc/modules/congestion_controller/include/send_side_congestion_controller.h
+++ b/webrtc/modules/congestion_controller/include/send_side_congestion_controller.h
@@ -57,6 +57,7 @@ class SendSideCongestionController : public CallStatsObserver,
    protected:
     virtual ~Observer() {}
   };
+  // TODO(nisse): Consider deleting the |observer| argument to constructors.
   SendSideCongestionController(const Clock* clock,
                                Observer* observer,
                                RtcEventLog* event_log,
@@ -70,6 +71,10 @@ class SendSideCongestionController : public CallStatsObserver,
   void RegisterPacketFeedbackObserver(PacketFeedbackObserver* observer);
   void DeRegisterPacketFeedbackObserver(PacketFeedbackObserver* observer);
 
+  // Currently, there can be at most one observer.
+  void RegisterNetworkObserver(Observer* observer);
+  void DeRegisterNetworkObserver(Observer* observer);
+
   virtual void SetBweBitrates(int min_bitrate_bps,
                               int start_bitrate_bps,
                               int max_bitrate_bps);
@@ -130,7 +135,8 @@ class SendSideCongestionController : public CallStatsObserver,
                                            uint8_t fraction_loss,
                                            int64_t rtt);
   const Clock* const clock_;
-  Observer* const observer_;
+  rtc::CriticalSection observer_lock_;
+  Observer* observer_ GUARDED_BY(observer_lock_);
   RtcEventLog* const event_log_;
   const std::unique_ptr<PacedSender> pacer_;
   const std::unique_ptr<BitrateController> bitrate_controller_;
diff --git a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
index e0918d7b76..e3f84d0e53 100644
--- a/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
+++ b/webrtc/modules/congestion_controller/send_side_congestion_controller.cc
@@ -93,6 +93,19 @@ void SendSideCongestionController::DeRegisterPacketFeedbackObserver(
   transport_feedback_adapter_.DeRegisterPacketFeedbackObserver(observer);
 }
 
+void SendSideCongestionController::RegisterNetworkObserver(Observer* observer) {
+  rtc::CritScope cs(&observer_lock_);
+  RTC_DCHECK(observer_ == nullptr);
+  observer_ = observer;
+}
+
+void SendSideCongestionController::DeRegisterNetworkObserver(
+    Observer* observer) {
+  rtc::CritScope cs(&observer_lock_);
+  RTC_DCHECK_EQ(observer_, observer);
+  observer_ = nullptr;
+}
+
 void SendSideCongestionController::SetBweBitrates(int min_bitrate_bps,
                                                   int start_bitrate_bps,
                                                   int max_bitrate_bps) {
@@ -245,11 +258,6 @@ SendSideCongestionController::GetTransportFeedbackVector() const {
 }
 
 void SendSideCongestionController::MaybeTriggerOnNetworkChanged() {
-  // TODO(perkj): |observer_| can be nullptr if the ctor that accepts a
-  // BitrateObserver is used. Remove this check once the ctor is removed.
-  if (!observer_)
-    return;
-
   uint32_t bitrate_bps;
   uint8_t fraction_loss;
   int64_t rtt;
@@ -269,8 +277,13 @@ void SendSideCongestionController::MaybeTriggerOnNetworkChanged() {
       rtc::CritScope cs(&bwe_lock_);
       probing_interval_ms = delay_based_bwe_->GetProbingIntervalMs();
     }
-    observer_->OnNetworkChanged(bitrate_bps, fraction_loss, rtt,
-                                probing_interval_ms);
+    {
+      rtc::CritScope cs(&observer_lock_);
+      if (observer_) {
+        observer_->OnNetworkChanged(bitrate_bps, fraction_loss, rtt,
+                                    probing_interval_ms);
+      }
+    }
   }
 }