diff --git a/net/dcsctp/rx/reassembly_queue.cc b/net/dcsctp/rx/reassembly_queue.cc
index 7183323642..36ade9230a 100644
--- a/net/dcsctp/rx/reassembly_queue.cc
+++ b/net/dcsctp/rx/reassembly_queue.cc
@@ -221,14 +221,21 @@ void ReassemblyQueue::AddReassembledMessage(
   }
 
   // With new TSNs in delivered_tsns, gaps might be filled.
+  MaybeMoveLastAssembledWatermarkFurther();
+
+  reassembled_messages_.emplace_back(std::move(message));
+}
+
+void ReassemblyQueue::MaybeMoveLastAssembledWatermarkFurther() {
+  // `delivered_tsns_` contain TSNS when there is a gap between ranges of
+  // assembled TSNs. `last_assembled_tsn_watermark_` should not be adjacent to
+  // that list, because if so, it can be moved.
   while (!delivered_tsns_.empty() &&
          *delivered_tsns_.begin() ==
              last_assembled_tsn_watermark_.next_value()) {
     last_assembled_tsn_watermark_.Increment();
     delivered_tsns_.erase(delivered_tsns_.begin());
   }
-
-  reassembled_messages_.emplace_back(std::move(message));
 }
 
 void ReassemblyQueue::Handle(const AnyForwardTsnChunk& forward_tsn) {
@@ -239,12 +246,21 @@ void ReassemblyQueue::Handle(const AnyForwardTsnChunk& forward_tsn) {
   delivered_tsns_.erase(delivered_tsns_.begin(),
                         delivered_tsns_.upper_bound(tsn));
 
+  MaybeMoveLastAssembledWatermarkFurther();
+
   queued_bytes_ -=
       streams_->HandleForwardTsn(tsn, forward_tsn.skipped_streams());
   RTC_DCHECK(IsConsistent());
 }
 
 bool ReassemblyQueue::IsConsistent() const {
+  // `delivered_tsns_` and `last_assembled_tsn_watermark_` mustn't overlap or be
+  // adjacent.
+  if (!delivered_tsns_.empty() &&
+      last_assembled_tsn_watermark_.next_value() >= *delivered_tsns_.begin()) {
+    return false;
+  }
+
   // Allow queued_bytes_ to be larger than max_size_bytes, as it's not actively
   // enforced in this class. This comparison will still trigger if queued_bytes_
   // became "negative".
diff --git a/net/dcsctp/rx/reassembly_queue.h b/net/dcsctp/rx/reassembly_queue.h
index 2fa11e3f75..9cc0c61eb6 100644
--- a/net/dcsctp/rx/reassembly_queue.h
+++ b/net/dcsctp/rx/reassembly_queue.h
@@ -128,6 +128,7 @@ class ReassemblyQueue {
   bool IsConsistent() const;
   void AddReassembledMessage(rtc::ArrayView<const UnwrappedTSN> tsns,
                              DcSctpMessage message);
+  void MaybeMoveLastAssembledWatermarkFurther();
 
   struct DeferredResetStreams {
     explicit DeferredResetStreams(OutgoingSSNResetRequestParameter req)