diff --git a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/UnionGenerator.java b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/UnionGenerator.java
index b3beb2cf3..4034c2732 100644
--- a/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/UnionGenerator.java
+++ b/codegen/core/src/main/java/software/amazon/smithy/python/codegen/generators/UnionGenerator.java
@@ -144,11 +144,10 @@ private void generateDeserializer() {
         writer.addImport("smithy_core.deserializers", "ShapeDeserializer");
         writer.addImport("smithy_core.exceptions", "SerializationError");
 
-        // TODO: add in unknown handling
-
         var symbol = symbolProvider.toSymbol(shape);
         var deserializerSymbol = symbol.expectProperty(SymbolProperties.DESERIALIZER);
         var schemaSymbol = symbol.expectProperty(SymbolProperties.SCHEMA);
+        var unknownSymbol = symbol.expectProperty(SymbolProperties.UNION_UNKNOWN);
         writer.putContext("schema", schemaSymbol);
         writer.write("""
                 class $1L:
@@ -168,6 +167,7 @@ def _consumer(self, schema: Schema, de: ShapeDeserializer) -> None:
                             ${4C|}
                             case _:
                                 logger.debug("Unexpected member schema: %s", schema)
+                                self._set_result($5L(tag=schema.expect_member_name()))
 
                     def _set_result(self, value: $2T) -> None:
                         if self._result is not None:
@@ -177,7 +177,8 @@ raise SerializationError("Unions must have exactly one value, but found more tha
                 deserializerSymbol.getName(),
                 symbol,
                 schemaSymbol,
-                writer.consumer(w -> deserializeMembers()));
+                writer.consumer(w -> deserializeMembers()),
+                unknownSymbol.getName());
     }
 
     private void deserializeMembers() {
diff --git a/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/_private/deserializers.py b/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/_private/deserializers.py
index f52791450..3cb91d383 100644
--- a/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/_private/deserializers.py
+++ b/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/_private/deserializers.py
@@ -50,7 +50,7 @@ def read_struct(
                     message_deserializer = self._create_deserializer(schema, headers)
                     message_deserializer.read_struct(schema, consumer)
                 else:
-                    member_schema = schema.members[member_name]
+                    member_schema = self._resolve_member_schema(schema, member_name)
                     message_deserializer = self._create_deserializer(
                         member_schema, headers
                     )
@@ -71,6 +71,21 @@ def read_struct(
             case _:
                 raise EventError(f"Unknown event structure: {self._event}")
 
+    def _resolve_member_schema(self, schema: Schema, member_name: str) -> Schema:
+        if member_schema := schema.members.get(member_name):
+            return member_schema
+
+        logger.debug(
+            "Received unmodeled event stream member %s for union %s",
+            member_name,
+            schema.id,
+        )
+        return Schema.member(
+            id=schema.id.with_member(member_name),
+            target=schema,
+            index=-1,
+        )
+
     def _create_deserializer(
         self, schema: Schema, headers: HEADERS_DICT
     ) -> ShapeDeserializer:
diff --git a/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/aio/__init__.py b/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/aio/__init__.py
index 58ed7f184..401442130 100644
--- a/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/aio/__init__.py
+++ b/packages/smithy-aws-event-stream/src/smithy_aws_event_stream/aio/__init__.py
@@ -132,8 +132,9 @@ async def receive(self) -> E | None:
         )
         result = self._deserializer(deserializer)
         logger.debug("Successfully deserialized event: %s", result)
-        if isinstance(getattr(result, "value"), Exception):
-            raise result.value  # type: ignore
+        value = getattr(result, "value", None)
+        if isinstance(value, Exception):
+            raise value
         return result
 
     async def close(self) -> None:
diff --git a/packages/smithy-aws-event-stream/tests/unit/_private/__init__.py b/packages/smithy-aws-event-stream/tests/unit/_private/__init__.py
index 09162ecce..1fbb8cd02 100644
--- a/packages/smithy-aws-event-stream/tests/unit/_private/__init__.py
+++ b/packages/smithy-aws-event-stream/tests/unit/_private/__init__.py
@@ -381,7 +381,7 @@ def serialize_members(self, serializer: ShapeSerializer):
 
 
 @dataclass
-class EventStreamUnknownEvent:
+class EventStreamUnknown:
     tag: str
 
     def serialize(self, serializer: ShapeSerializer):
@@ -396,7 +396,7 @@ def serialize_members(self, serializer: ShapeSerializer):
     | EventStreamPayloadEvent
     | EventStreamBlobPayloadEvent
     | EventStreamErrorEvent
-    | EventStreamUnknownEvent
+    | EventStreamUnknown
 )
 
 
@@ -429,7 +429,7 @@ def _consumer(self, schema: Schema, de: ShapeDeserializer) -> None:
                 self._set_result(EventStreamErrorEvent(ErrorEvent.deserialize(de)))
 
             case _:
-                raise SmithyError(f"Unexpected member schema: {schema}")
+                self._set_result(EventStreamUnknown(tag=schema.expect_member_name()))
 
     def _set_result(self, value: EventStream) -> None:
         if self._result is not None:
@@ -635,6 +635,19 @@ def _consumer(schema: Schema, de: ShapeDeserializer) -> None:
 ]
 
 
+UNKNOWN_EVENT_CASE = (
+    EventStreamUnknown(tag="unmodeledEvent"),
+    EventMessage(
+        headers={
+            ":message-type": "event",
+            ":event-type": "unmodeledEvent",
+            ":content-type": "application/json",
+        },
+        payload=b"{}",
+    ),
+)
+
+
 INITIAL_REQUEST_CASE = (
     EventStreamOperationInputOutput(message="The initial request!"),
     EventMessage(
diff --git a/packages/smithy-aws-event-stream/tests/unit/_private/test_deserializers.py b/packages/smithy-aws-event-stream/tests/unit/_private/test_deserializers.py
index 34081bbf8..7ae9c77ce 100644
--- a/packages/smithy-aws-event-stream/tests/unit/_private/test_deserializers.py
+++ b/packages/smithy-aws-event-stream/tests/unit/_private/test_deserializers.py
@@ -20,6 +20,7 @@
     EventStreamDeserializer,
     EventStreamErrorEvent,
     EventStreamOperationInputOutput,
+    EventStreamUnknown,
 )
 
 
@@ -126,3 +127,20 @@ async def test_read_closed_receiver_source() -> None:
     with pytest.raises(IOError):
         await receiver.receive()
     assert receiver.closed
+
+
+def test_deserialize_unknown_event_type():
+    message = EventMessage(
+        headers={
+            ":message-type": "event",
+            ":event-type": "unmodeledEvent",
+            ":content-type": "application/json",
+        },
+        payload=b"{}",
+    )
+    source = Event.decode(BytesIO(message.encode()))
+    assert source is not None
+    deserializer = EventDeserializer(event=source, payload_codec=JSONCodec())
+    result = EventStreamDeserializer().deserialize(deserializer)
+    assert isinstance(result, EventStreamUnknown)
+    assert result.tag == "unmodeledEvent"