diff --git a/runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerde.java b/runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerde.java
new file mode 100644
index 000000000000..dd30ed3696eb
--- /dev/null
+++ b/runners/kafka-streams/src/main/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerde.java
@@ -0,0 +1,121 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.runners.kafka.streams.translation;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.DataInputStream;
+import java.io.DataOutputStream;
+import java.io.IOException;
+import org.apache.beam.sdk.coders.Coder;
+import org.apache.beam.sdk.values.WindowedValue;
+import org.apache.kafka.common.errors.SerializationException;
+import org.apache.kafka.common.serialization.Deserializer;
+import org.apache.kafka.common.serialization.Serde;
+import org.apache.kafka.common.serialization.Serializer;
+
+/**
+ * Kafka {@link Serde} for {@link KStreamsPayload}, enabling the envelope to cross topic boundaries
+ * (e.g. the repartition topic a {@code GroupByKey} introduces). Until now {@link KStreamsPayload}
+ * only flowed in-JVM via {@code ProcessorContext#forward}, so no serialization was needed.
+ *
+ * <p>The wire format is a one-byte discriminator followed by the variant body:
+ *
+ * <ul>
+ *   <li><b>data</b>: {@code [0x00][windowedValueCoder-encoded WindowedValue]} — the data element is
+ *       encoded with the {@link Coder} supplied for the topic's PCollection.
+ *   <li><b>watermark</b>: {@code [0x01][long watermarkMillis][int sourcePartition][int
+ *       totalSourcePartitions]} — the in-band watermark report, coder-independent.
+ * </ul>
+ *
+ * <p>A {@link KStreamsPayloadSerde} is parameterized by the {@link Coder} for the data variant
+ * because different topics carry different element types; the watermark variant needs no coder.
+ *
+ * <p>The serde assumes non-null payloads: the topics it is used on (repartition and watermark
+ * fan-out) are not log-compacted, so no tombstone (null-valued) records occur.
+ *
+ * @param <T> the data element type carried by data payloads on this topic
+ */
+public final class KStreamsPayloadSerde<T> implements Serde<KStreamsPayload<T>> {
+
+  private static final byte DATA_TAG = 0;
+  private static final byte WATERMARK_TAG = 1;
+
+  private final Coder<WindowedValue<T>> dataCoder;
+
+  public KStreamsPayloadSerde(Coder<WindowedValue<T>> dataCoder) {
+    this.dataCoder = dataCoder;
+  }
+
+  @Override
+  public Serializer<KStreamsPayload<T>> serializer() {
+    return new PayloadSerializer();
+  }
+
+  @Override
+  public Deserializer<KStreamsPayload<T>> deserializer() {
+    return new PayloadDeserializer();
+  }
+
+  private final class PayloadSerializer implements Serializer<KStreamsPayload<T>> {
+    @Override
+    public byte[] serialize(String topic, KStreamsPayload<T> payload) {
+      ByteArrayOutputStream out = new ByteArrayOutputStream();
+      try {
+        if (payload.isData()) {
+          out.write(DATA_TAG);
+          dataCoder.encode(payload.getData(), out);
+        } else {
+          WatermarkPayload watermark = payload.asWatermark();
+          DataOutputStream dataOut = new DataOutputStream(out);
+          dataOut.writeByte(WATERMARK_TAG);
+          dataOut.writeLong(watermark.getWatermarkMillis());
+          dataOut.writeInt(watermark.getSourcePartition());
+          dataOut.writeInt(watermark.getTotalSourcePartitions());
+          dataOut.flush();
+        }
+      } catch (IOException e) {
+        throw new SerializationException("Failed to serialize KStreamsPayload", e);
+      }
+      return out.toByteArray();
+    }
+  }
+
+  private final class PayloadDeserializer implements Deserializer<KStreamsPayload<T>> {
+    @Override
+    public KStreamsPayload<T> deserialize(String topic, byte[] bytes) {
+      ByteArrayInputStream in = new ByteArrayInputStream(bytes);
+      try {
+        int tag = in.read();
+        if (tag == DATA_TAG) {
+          return KStreamsPayload.data(dataCoder.decode(in));
+        } else if (tag == WATERMARK_TAG) {
+          DataInputStream dataIn = new DataInputStream(in);
+          long watermarkMillis = dataIn.readLong();
+          int sourcePartition = dataIn.readInt();
+          int totalSourcePartitions = dataIn.readInt();
+          return KStreamsPayload.watermark(watermarkMillis, sourcePartition, totalSourcePartitions);
+        } else {
+          throw new SerializationException("Unknown KStreamsPayload tag: " + tag);
+        }
+      } catch (IOException e) {
+        throw new SerializationException("Failed to deserialize KStreamsPayload", e);
+      }
+    }
+  }
+}
diff --git a/runners/kafka-streams/src/test/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerdeTest.java b/runners/kafka-streams/src/test/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerdeTest.java
new file mode 100644
index 000000000000..1e4c4c0263a4
--- /dev/null
+++ b/runners/kafka-streams/src/test/java/org/apache/beam/runners/kafka/streams/translation/KStreamsPayloadSerdeTest.java
@@ -0,0 +1,85 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.runners.kafka.streams.translation;
+
+import static org.hamcrest.CoreMatchers.is;
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.junit.Assert.assertThrows;
+
+import org.apache.beam.sdk.coders.Coder;
+import org.apache.beam.sdk.coders.VarIntCoder;
+import org.apache.beam.sdk.transforms.windowing.BoundedWindow;
+import org.apache.beam.sdk.transforms.windowing.GlobalWindow;
+import org.apache.beam.sdk.values.WindowedValue;
+import org.apache.beam.sdk.values.WindowedValues;
+import org.apache.kafka.common.errors.SerializationException;
+import org.apache.kafka.common.serialization.Deserializer;
+import org.apache.kafka.common.serialization.Serializer;
+import org.junit.Test;
+
+/** Tests for {@link KStreamsPayloadSerde}. */
+public class KStreamsPayloadSerdeTest {
+
+  private static final String TOPIC = "ks-payload-serde-test";
+
+  private final Coder<WindowedValue<Integer>> dataCoder =
+      WindowedValues.getFullCoder(VarIntCoder.of(), GlobalWindow.Coder.INSTANCE);
+  private final KStreamsPayloadSerde<Integer> serde = new KStreamsPayloadSerde<>(dataCoder);
+
+  private KStreamsPayload<Integer> roundTrip(KStreamsPayload<Integer> payload) {
+    Serializer<KStreamsPayload<Integer>> serializer = serde.serializer();
+    Deserializer<KStreamsPayload<Integer>> deserializer = serde.deserializer();
+    return deserializer.deserialize(TOPIC, serializer.serialize(TOPIC, payload));
+  }
+
+  @Test
+  public void roundTripsDataPayload() {
+    KStreamsPayload<Integer> payload = KStreamsPayload.data(WindowedValues.valueInGlobalWindow(42));
+    KStreamsPayload<Integer> out = roundTrip(payload);
+    assertThat(out.isData(), is(true));
+    assertThat(out.getData().getValue(), is(42));
+    assertThat(out, is(payload));
+  }
+
+  @Test
+  public void roundTripsWatermarkPayload() {
+    KStreamsPayload<Integer> payload = KStreamsPayload.watermark(12345L, 2, 4);
+    KStreamsPayload<Integer> out = roundTrip(payload);
+    assertThat(out.isWatermark(), is(true));
+    assertThat(out.asWatermark().getWatermarkMillis(), is(12345L));
+    assertThat(out.asWatermark().getSourcePartition(), is(2));
+    assertThat(out.asWatermark().getTotalSourcePartitions(), is(4));
+    assertThat(out, is(payload));
+  }
+
+  @Test
+  public void roundTripsTerminalMaxWatermark() {
+    KStreamsPayload<Integer> payload =
+        KStreamsPayload.watermark(BoundedWindow.TIMESTAMP_MAX_VALUE.getMillis(), 0, 1);
+    assertThat(
+        roundTrip(payload).asWatermark().getWatermarkMillis(),
+        is(BoundedWindow.TIMESTAMP_MAX_VALUE.getMillis()));
+  }
+
+  @Test
+  public void unknownTagThrows() {
+    byte[] bogus = new byte[] {(byte) 0x7f};
+    assertThrows(
+        SerializationException.class, () -> serde.deserializer().deserialize(TOPIC, bogus));
+  }
+}