diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl2.cc b/modules/rtp_rtcp/source/rtp_rtcp_impl2.cc
index 136c11c..ec42f61 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl2.cc
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl2.cc
@@ -78,6 +78,7 @@
 void ModuleRtpRtcpImpl2::RtpSenderContext::AssignSequenceNumber(
     RtpPacketToSend* packet) {
   if (deferred_sequencing_) {
+    MutexLock lock(&mutex_sequencer_);
     sequencer_.Sequence(*packet);
   } else {
     packet_generator.AssignSequenceNumber(packet);
@@ -187,6 +188,7 @@
 
 uint16_t ModuleRtpRtcpImpl2::SequenceNumber() const {
   if (rtp_sender_->deferred_sequencing_) {
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     return rtp_sender_->sequencer_.media_sequence_number();
   }
   return rtp_sender_->packet_generator.SequenceNumber();
@@ -196,6 +198,7 @@
 void ModuleRtpRtcpImpl2::SetSequenceNumber(const uint16_t seq_num) {
   if (rtp_sender_->deferred_sequencing_) {
     RTC_DCHECK_RUN_ON(&pacer_thread_checker_);
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     if (rtp_sender_->sequencer_.media_sequence_number() != seq_num) {
       rtp_sender_->sequencer_.set_media_sequence_number(seq_num);
       rtp_sender_->packet_history.Clear();
@@ -208,6 +211,7 @@
 void ModuleRtpRtcpImpl2::SetRtpState(const RtpState& rtp_state) {
   rtp_sender_->packet_generator.SetRtpState(rtp_state);
   if (rtp_sender_->deferred_sequencing_) {
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     rtp_sender_->sequencer_.SetRtpState(rtp_state);
   }
   rtcp_sender_.SetTimestampOffset(rtp_state.start_timestamp);
@@ -216,6 +220,7 @@
 void ModuleRtpRtcpImpl2::SetRtxState(const RtpState& rtp_state) {
   rtp_sender_->packet_generator.SetRtxRtpState(rtp_state);
   if (rtp_sender_->deferred_sequencing_) {
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     rtp_sender_->sequencer_.set_rtx_sequence_number(rtp_state.sequence_number);
   }
 }
@@ -223,6 +228,7 @@
 RtpState ModuleRtpRtcpImpl2::GetRtpState() const {
   RtpState state = rtp_sender_->packet_generator.GetRtpState();
   if (rtp_sender_->deferred_sequencing_) {
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     rtp_sender_->sequencer_.PopulateRtpState(state);
   }
   return state;
@@ -231,6 +237,7 @@
 RtpState ModuleRtpRtcpImpl2::GetRtxState() const {
   RtpState state = rtp_sender_->packet_generator.GetRtxRtpState();
   if (rtp_sender_->deferred_sequencing_) {
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     state.sequence_number = rtp_sender_->sequencer_.rtx_sequence_number();
   }
   return state;
@@ -387,6 +394,7 @@
     if (!rtp_sender_->packet_generator.SendingMedia()) {
       return false;
     }
+    MutexLock lock(&rtp_sender_->mutex_sequencer_);
     if (packet->packet_type() == RtpPacketMediaType::kPadding &&
         packet->Ssrc() == rtp_sender_->packet_generator.SSRC() &&
         !rtp_sender_->sequencer_.CanSendPaddingOnMediaSsrc()) {
@@ -452,11 +460,8 @@
 std::vector<std::unique_ptr<RtpPacketToSend>>
 ModuleRtpRtcpImpl2::GeneratePadding(size_t target_size_bytes) {
   RTC_DCHECK(rtp_sender_);
-
-  // `can_send_padding_on_media_ssrc` set to false but is ignored at this
-  // point, RTPSender will internally query `sequencer_` while holding the
-  // send lock.
   RTC_DCHECK_RUN_ON(&pacer_thread_checker_);
+  MutexLock lock(&rtp_sender_->mutex_sequencer_);
 
   // `can_send_padding_on_media_ssrc` set to false when deferred sequencing
   // is off. It will be ignored in that case, RTPSender will internally query
diff --git a/modules/rtp_rtcp/source/rtp_rtcp_impl2.h b/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
index 14d1409..b468bb7 100644
--- a/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
+++ b/modules/rtp_rtcp/source/rtp_rtcp_impl2.h
@@ -269,8 +269,12 @@
     // If false, sequencing is owned by `packet_generator` and can happen on
     // several threads. If true, sequencing always happens on the pacer thread.
     const bool deferred_sequencing_;
+    // TODO(bugs.webrtc.org/11340): Remove lock one we can guarantee that
+    // setting/getting rtp state only happens after removal from packet sending
+    // code path.
+    mutable Mutex mutex_sequencer_;
     // Handles sequence number assignment and padding timestamp generation.
-    PacketSequencer sequencer_;
+    PacketSequencer sequencer_ RTC_GUARDED_BY(mutex_sequencer_);
     // Handles final time timestamping/stats/etc and handover to Transport.
     RtpSenderEgress packet_sender;
     // If no paced sender configured, this class will be used to pass packets