diff --git a/modules/rtp_rtcp/source/rtp_sender.cc b/modules/rtp_rtcp/source/rtp_sender.cc
index 9f10c40e57..c79f33d089 100644
--- a/modules/rtp_rtcp/source/rtp_sender.cc
+++ b/modules/rtp_rtcp/source/rtp_sender.cc
@@ -619,10 +619,13 @@ size_t RTPSender::SendPadData(size_t bytes,
     PacketOptions options;
     // Padding packets are never retransmissions.
     options.is_retransmit = false;
-    bool has_transport_seq_num =
-        UpdateTransportSequenceNumber(&padding_packet, &options.packet_id);
+    bool has_transport_seq_num;
+    {
+      rtc::CritScope lock(&send_critsect_);
+      has_transport_seq_num =
+          UpdateTransportSequenceNumber(&padding_packet, &options.packet_id);
+    }
     padding_packet.SetPadding(padding_bytes_in_packet, &random_);
-
     if (has_transport_seq_num) {
       AddPacketToTransportFeedback(options.packet_id, padding_packet,
                                    pacing_info);
@@ -838,7 +841,13 @@ bool RTPSender::PrepareAndSendPacket(std::unique_ptr<RtpPacketToSend> packet,
   // E.g. RTPSender::TrySendRedundantPayloads calls PrepareAndSendPacket with
   // send_over_rtx = true but is_retransmit = false.
   options.is_retransmit = is_retransmit || send_over_rtx;
-  if (UpdateTransportSequenceNumber(packet_to_send, &options.packet_id)) {
+  bool has_transport_seq_num;
+  {
+    rtc::CritScope lock(&send_critsect_);
+    has_transport_seq_num =
+        UpdateTransportSequenceNumber(packet_to_send, &options.packet_id);
+  }
+  if (has_transport_seq_num) {
     AddPacketToTransportFeedback(options.packet_id, *packet_to_send,
                                  pacing_info);
   }
@@ -971,7 +980,14 @@ bool RTPSender::SendToNetwork(std::unique_ptr<RtpPacketToSend> packet,
 
   PacketOptions options;
   options.is_retransmit = false;
-  if (UpdateTransportSequenceNumber(packet.get(), &options.packet_id)) {
+
+  bool has_transport_seq_num;
+  {
+    rtc::CritScope lock(&send_critsect_);
+    has_transport_seq_num =
+        UpdateTransportSequenceNumber(packet.get(), &options.packet_id);
+  }
+  if (has_transport_seq_num) {
     AddPacketToTransportFeedback(options.packet_id, *packet.get(),
                                  PacedPacketInfo());
   }
@@ -1181,10 +1197,9 @@ bool RTPSender::AssignSequenceNumber(RtpPacketToSend* packet) {
 }
 
 bool RTPSender::UpdateTransportSequenceNumber(RtpPacketToSend* packet,
-                                              int* packet_id) const {
+                                              int* packet_id) {
   RTC_DCHECK(packet);
   RTC_DCHECK(packet_id);
-  rtc::CritScope lock(&send_critsect_);
   if (!rtp_header_extension_map_.IsRegistered(TransportSequenceNumber::kId))
     return false;
 
diff --git a/modules/rtp_rtcp/source/rtp_sender.h b/modules/rtp_rtcp/source/rtp_sender.h
index 5a93d7a520..1adf6fe8c7 100644
--- a/modules/rtp_rtcp/source/rtp_sender.h
+++ b/modules/rtp_rtcp/source/rtp_sender.h
@@ -247,8 +247,8 @@ class RTPSender {
                           int64_t capture_time_ms,
                           uint32_t ssrc);
 
-  bool UpdateTransportSequenceNumber(RtpPacketToSend* packet,
-                                     int* packet_id) const;
+  bool UpdateTransportSequenceNumber(RtpPacketToSend* packet, int* packet_id)
+      RTC_EXCLUSIVE_LOCKS_REQUIRED(send_critsect_);
 
   void UpdateRtpStats(const RtpPacketToSend& packet,
                       bool is_rtx,