diff --git a/sdk/objc/api/peerconnection/RTCConfiguration.h b/sdk/objc/api/peerconnection/RTCConfiguration.h
index 4356b8d494..c7a20533a3 100644
--- a/sdk/objc/api/peerconnection/RTCConfiguration.h
+++ b/sdk/objc/api/peerconnection/RTCConfiguration.h
@@ -63,8 +63,15 @@ typedef NS_ENUM(NSInteger, RTCEncryptionKeyType) {
 
 /** Represents the chosen SDP semantics for the RTCPeerConnection. */
 typedef NS_ENUM(NSInteger, RTCSdpSemantics) {
+  // TODO(https://crbug.com/webrtc/13528): Remove support for Plan B.
   RTCSdpSemanticsPlanB,
   RTCSdpSemanticsUnifiedPlan,
+  // The default sdpSemantics value is about to change to Unified Plan. During
+  // a short transition period, NotSpecified is used to ensure clients that
+  // don't set sdpSemantics are aware of the change by CHECK-crashing.
+  // TODO(https://crbug.com/webrtc/11121): When the default has changed to
+  // UnifiedPlan, delete NotSpecified.
+  RTCSdpSemanticsNotSpecified,
 };
 
 NS_ASSUME_NONNULL_BEGIN
@@ -161,9 +168,10 @@ RTC_OBJC_EXPORT
  */
 @property(nonatomic, copy, nullable) NSNumber *iceCheckMinInterval;
 
-/** Configure the SDP semantics used by this PeerConnection. Note that the
- *  WebRTC 1.0 specification requires UnifiedPlan semantics. The
- *  RTCRtpTransceiver API is only available with UnifiedPlan semantics.
+/** Configure the SDP semantics used by this PeerConnection. The WebRTC 1.0
+ *  specification requires RTCSdpSemanticsUnifiedPlan semantics and the
+ *  RtpTransceiver API is only available in Unified Plan. RTCSdpSemanticsPlanB
+ *  is being deprecated and will be removed at a future date.
  *
  *  PlanB will cause RTCPeerConnection to create offers and answers with at
  *  most one audio and one video m= section with multiple RTCRtpSenders and
@@ -174,14 +182,18 @@ RTC_OBJC_EXPORT
  *  UnifiedPlan will cause RTCPeerConnection to create offers and answers with
  *  multiple m= sections where each m= section maps to one RTCRtpSender and one
  *  RTCRtpReceiver (an RTCRtpTransceiver), either both audio or both
- *  video. This will also cause RTCPeerConnection) to ignore all but the first a=ssrc
- *  lines that form a Plan B stream.
+ *  video. This will also cause RTCPeerConnection) to ignore all but the first
+ *  a=ssrc lines that form a Plan B stream.
  *
- *  For users who wish to send multiple audio/video streams and need to stay
- *  interoperable with legacy WebRTC implementations or use legacy APIs,
- *  specify PlanB.
+ *  For users who have to interwork with legacy WebRTC implementations, it
+ *  is possible to specify PlanB until the code is finally removed
+ *  (https://crbug.com/webrtc/13528).
  *
- *  For all other users, specify UnifiedPlan.
+ *  The default SdpSemantics value is about to change to UnifiedPlan. During a
+ *  short transition period, NotSpecified is used to ensure clients that don't
+ *  set SdpSemantics are aware of the change by CHECK-crashing.
+ *  TODO(https://crbug.com/webrtc/11121): When the default has changed to
+ *  UnifiedPlan, delete NotSpecified.
  */
 @property(nonatomic, assign) RTCSdpSemantics sdpSemantics;
 
diff --git a/sdk/objc/api/peerconnection/RTCConfiguration.mm b/sdk/objc/api/peerconnection/RTCConfiguration.mm
index 0885e96f4e..9e9bca7745 100644
--- a/sdk/objc/api/peerconnection/RTCConfiguration.mm
+++ b/sdk/objc/api/peerconnection/RTCConfiguration.mm
@@ -68,7 +68,7 @@
 - (instancetype)init {
   // Copy defaults.
   webrtc::PeerConnectionInterface::RTCConfiguration config;
-  config.sdp_semantics = webrtc::SdpSemantics::kPlanB_DEPRECATED;
+  config.sdp_semantics = webrtc::SdpSemantics::kNotSpecified;
   return [self initWithNativeConfiguration:config];
 }
 
@@ -525,6 +525,8 @@
       return webrtc::SdpSemantics::kPlanB_DEPRECATED;
     case RTCSdpSemanticsUnifiedPlan:
       return webrtc::SdpSemantics::kUnifiedPlan;
+    case RTCSdpSemanticsNotSpecified:
+      return webrtc::SdpSemantics::kNotSpecified;
   }
 }
 
@@ -535,8 +537,7 @@
     case webrtc::SdpSemantics::kUnifiedPlan:
       return RTCSdpSemanticsUnifiedPlan;
     case webrtc::SdpSemantics::kNotSpecified:
-      RTC_DCHECK_NOTREACHED();
-      return RTCSdpSemanticsUnifiedPlan;
+      return RTCSdpSemanticsNotSpecified;
   }
 }
 
@@ -546,6 +547,8 @@
       return @"PLAN_B";
     case RTCSdpSemanticsUnifiedPlan:
       return @"UNIFIED_PLAN";
+    case RTCSdpSemanticsNotSpecified:
+      return @"NOT_SPECIFIED";
   }
 }
 
diff --git a/sdk/objc/unittests/RTCPeerConnectionTest.mm b/sdk/objc/unittests/RTCPeerConnectionTest.mm
index 19b3c8e69f..8ebe6271ab 100644
--- a/sdk/objc/unittests/RTCPeerConnectionTest.mm
+++ b/sdk/objc/unittests/RTCPeerConnectionTest.mm
@@ -42,6 +42,7 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithURLStrings:urlStrings];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   config.iceServers = @[ server ];
   config.iceTransportPolicy = RTCIceTransportPolicyRelay;
   config.bundlePolicy = RTCBundlePolicyMaxBundle;
@@ -121,6 +122,7 @@
       [[RTC_OBJC_TYPE(RTCIceServer) alloc] initWithURLStrings:urlStrings];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   config.iceServers = @[ server ];
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
@@ -145,6 +147,7 @@
       [[RTC_OBJC_TYPE(RTCPeerConnectionFactory) alloc] init];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
                                                            optionalConstraints:nil];
@@ -175,6 +178,7 @@
       [[RTC_OBJC_TYPE(RTCPeerConnectionFactory) alloc] init];
 
   RTC_OBJC_TYPE(RTCConfiguration) *config = [[RTC_OBJC_TYPE(RTCConfiguration) alloc] init];
+  config.sdpSemantics = RTCSdpSemanticsUnifiedPlan;
   RTC_OBJC_TYPE(RTCMediaConstraints) *contraints =
       [[RTC_OBJC_TYPE(RTCMediaConstraints) alloc] initWithMandatoryConstraints:@{}
                                                            optionalConstraints:nil];