diff --git a/pc/webrtc_sdp.cc b/pc/webrtc_sdp.cc
index da024eab81..8bd8d369cc 100644
--- a/pc/webrtc_sdp.cc
+++ b/pc/webrtc_sdp.cc
@@ -2614,6 +2614,25 @@ void MaybeCreateStaticPayloadAudioCodecs(const std::vector<int>& fmts,
   }
 }
 
+static void BackfillCodecParameters(std::vector<cricket::Codec>& codecs) {
+  for (auto& codec : codecs) {
+    std::string unused_value;
+    if (absl::EqualsIgnoreCase(cricket::kVp9CodecName, codec.name)) {
+      // https://datatracker.ietf.org/doc/html/draft-ietf-payload-vp9#section-6
+      // profile-id defaults to "0"
+      if (!codec.GetParam(cricket::kVP9ProfileId, &unused_value)) {
+        codec.SetParam(cricket::kVP9ProfileId, "0");
+      }
+    } else if (absl::EqualsIgnoreCase(cricket::kH264CodecName, codec.name)) {
+      // https://www.rfc-editor.org/rfc/rfc6184#section-6.2
+      // packetization-mode defaults to "0"
+      if (!codec.GetParam(cricket::kH264FmtpPacketizationMode, &unused_value)) {
+        codec.SetParam(cricket::kH264FmtpPacketizationMode, "0");
+      }
+    }
+  }
+}
+
 static std::unique_ptr<MediaContentDescription> ParseContentDescription(
     absl::string_view message,
     const cricket::MediaType media_type,
@@ -2657,6 +2676,9 @@ static std::unique_ptr<MediaContentDescription> ParseContentDescription(
                                                    const cricket::Codec& b) {
     return payload_type_preferences[a.id] > payload_type_preferences[b.id];
   });
+  // Backfill any default parameters.
+  BackfillCodecParameters(codecs);
+
   media_desc->set_codecs(codecs);
   return media_desc;
 }
diff --git a/pc/webrtc_sdp_unittest.cc b/pc/webrtc_sdp_unittest.cc
index ae26ba0ce2..bc79d174ac 100644
--- a/pc/webrtc_sdp_unittest.cc
+++ b/pc/webrtc_sdp_unittest.cc
@@ -5096,3 +5096,42 @@ TEST_F(WebRtcSdpTest, IgnoresUnknownAttributeLines) {
   JsepSessionDescription jdesc(kDummyType);
   EXPECT_TRUE(SdpDeserialize(sdp, &jdesc));
 }
+
+TEST_F(WebRtcSdpTest, BackfillsDefaultFmtpValues) {
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 0 3 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "a=group:BUNDLE 0\r\n"
+      "a=fingerprint:sha-1 "
+      "4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB\r\n"
+      "a=setup:actpass\r\n"
+      "a=ice-ufrag:ETEn\r\n"
+      "a=ice-pwd:OtSK0WpNtpUjkY4+86js7Z/l\r\n"
+      "m=video 9 UDP/TLS/RTP/SAVPF 96 97\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=rtcp-mux\r\n"
+      "a=sendonly\r\n"
+      "a=mid:0\r\n"
+      "a=rtpmap:96 H264/90000\r\n"
+      "a=rtpmap:97 VP9/90000\r\n"
+      "a=ssrc:1234 cname:test\r\n";
+  JsepSessionDescription jdesc(kDummyType);
+  EXPECT_TRUE(SdpDeserialize(sdp, &jdesc));
+  ASSERT_EQ(1u, jdesc.description()->contents().size());
+  const auto content = jdesc.description()->contents()[0];
+  const auto* description = content.media_description();
+  ASSERT_NE(description, nullptr);
+  const std::vector<cricket::Codec> codecs = description->codecs();
+  ASSERT_EQ(codecs.size(), 2u);
+  std::string value;
+
+  EXPECT_EQ(codecs[0].name, "H264");
+  EXPECT_TRUE(codecs[0].GetParam("packetization-mode", &value));
+  EXPECT_EQ(value, "0");
+
+  EXPECT_EQ(codecs[1].name, "VP9");
+  EXPECT_TRUE(codecs[1].GetParam("profile-id", &value));
+  EXPECT_EQ(value, "0");
+}