diff --git a/media/gpu/windows/d3d12_video_encode_h264_delegate.cc b/media/gpu/windows/d3d12_video_encode_h264_delegate.cc
--- a/media/gpu/windows/d3d12_video_encode_h264_delegate.cc
+++ b/media/gpu/windows/d3d12_video_encode_h264_delegate.cc
@@ -648,7 +648,13 @@ EncoderStatus D3D12VideoEncodeH264Delegate::InitializeVideoEncoder(
                                  "support manual reference control, got %u",
                                  picture_control_support_h264.MaxDPBCapacity)};
     }
-    max_num_ref_frames_ = picture_control_support_h264.MaxDPBCapacity;
+    // Manual reference control is implemented via H.264 long-term references,
+    // so the number of usable slots must not exceed the driver's
+    // MaxLongTermReferences. Reserve one extra DPB slot for frame_num gap
+    // handling (matches the SVC path), but never exceed MaxDPBCapacity.
+    max_num_ref_frames_ = std::min<uint32_t>(
+        picture_control_support_h264.MaxDPBCapacity,
+        picture_control_support_h264.MaxLongTermReferences + 1);
   }

   if ((config.bitrate.mode() == Bitrate::Mode::kConstant ||