diff --git a/sdk-actors/src/test/java/io/dapr/actors/runtime/ActorObjectSerializerTest.java b/sdk-actors/src/test/java/io/dapr/actors/runtime/ActorObjectSerializerTest.java
new file mode 100644
index 0000000000..a5178bf035
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+++ b/sdk-actors/src/test/java/io/dapr/actors/runtime/ActorObjectSerializerTest.java
@@ -0,0 +1,196 @@
+/*
+ * Copyright 2026 The Dapr Authors
+ * Licensed 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 io.dapr.actors.runtime;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.time.Duration;
+import java.util.Objects;
+
+/**
+ * Unit tests for {@link ActorObjectSerializer}.
+ */
+public class ActorObjectSerializerTest {
+
+  private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
+
+  private static final ActorObjectSerializer SERIALIZER = new ActorObjectSerializer();
+
+  public static class CustomState {
+
+    public String name;
+
+    public int count;
+
+    public CustomState() {
+    }
+
+    public CustomState(String name, int count) {
+      this.name = name;
+      this.count = count;
+    }
+
+    @Override
+    public boolean equals(Object obj) {
+      if (this == obj) {
+        return true;
+      }
+      if (!(obj instanceof CustomState)) {
+        return false;
+      }
+      CustomState other = (CustomState) obj;
+      return this.count == other.count && Objects.equals(this.name, other.name);
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(this.name, this.count);
+    }
+  }
+
+  @Test
+  public void serializeNullReturnsNull() throws IOException {
+    Assertions.assertNull(SERIALIZER.serialize(null));
+  }
+
+  @Test
+  public void deserializeNullActorTimerReturnsNull() throws IOException {
+    Assertions.assertNull(SERIALIZER.deserialize(null, ActorTimerParams.class));
+  }
+
+  @Test
+  public void deserializeNullActorReminderReturnsNull() throws IOException {
+    Assertions.assertNull(SERIALIZER.deserialize(null, ActorReminderParams.class));
+  }
+
+  @Test
+  public void serializeActorTimerWithData() throws IOException {
+    byte[] data = "hello world".getBytes(StandardCharsets.UTF_8);
+    ActorTimerParams timer = new ActorTimerParams(
+        "myCallback",
+        data,
+        Duration.ofMinutes(1),
+        Duration.ofMinutes(2));
+
+    byte[] serialized = SERIALIZER.serialize(timer);
+
+    String expected = "{\"dueTime\":\"0h1m0s0ms\",\"period\":\"0h2m0s0ms\","
+        + "\"callback\":\"myCallback\",\"data\":\"aGVsbG8gd29ybGQ=\"}";
+    Assertions.assertEquals(OBJECT_MAPPER.readTree(expected), OBJECT_MAPPER.readTree(serialized));
+  }
+
+  @Test
+  public void actorTimerRoundTrip() throws IOException {
+    byte[] data = "timer state".getBytes(StandardCharsets.UTF_8);
+    ActorTimerParams original = new ActorTimerParams(
+        "myCallback",
+        data,
+        Duration.ofMinutes(7).plusSeconds(17),
+        Duration.ofHours(1).plusSeconds(3));
+
+    byte[] serialized = SERIALIZER.serialize(original);
+    ActorTimerParams recreated = SERIALIZER.deserialize(serialized, ActorTimerParams.class);
+
+    Assertions.assertEquals(original.getCallback(), recreated.getCallback());
+    Assertions.assertEquals(original.getDueTime(), recreated.getDueTime());
+    Assertions.assertEquals(original.getPeriod(), recreated.getPeriod());
+    Assertions.assertArrayEquals(original.getData(), recreated.getData());
+  }
+
+  @Test
+  public void actorTimerRoundTripWithoutData() throws IOException {
+    ActorTimerParams original = new ActorTimerParams(
+        "myCallback",
+        null,
+        Duration.ofSeconds(30),
+        Duration.ofMinutes(5));
+
+    byte[] serialized = SERIALIZER.serialize(original);
+    ActorTimerParams recreated = SERIALIZER.deserialize(serialized, ActorTimerParams.class);
+
+    Assertions.assertEquals(original.getCallback(), recreated.getCallback());
+    Assertions.assertEquals(original.getDueTime(), recreated.getDueTime());
+    Assertions.assertEquals(original.getPeriod(), recreated.getPeriod());
+    Assertions.assertNull(recreated.getData());
+  }
+
+  @Test
+  public void deserializeActorTimerWithoutOptionalAttributes() throws IOException {
+    byte[] serialized = "{\"callback\":\"myCallback\"}".getBytes(StandardCharsets.UTF_8);
+
+    ActorTimerParams recreated = SERIALIZER.deserialize(serialized, ActorTimerParams.class);
+
+    Assertions.assertEquals("myCallback", recreated.getCallback());
+    Assertions.assertNull(recreated.getDueTime());
+    Assertions.assertNull(recreated.getPeriod());
+    Assertions.assertNull(recreated.getData());
+  }
+
+  @Test
+  public void serializeActorRuntimeConfigWithAllAttributes() throws IOException {
+    ActorRuntimeConfig config = new ActorRuntimeConfig()
+        .addRegisteredActorType("MyActor")
+        .setActorIdleTimeout(Duration.ofHours(1))
+        .setActorScanInterval(Duration.ofSeconds(30))
+        .setDrainOngoingCallTimeout(Duration.ofMinutes(1))
+        .setDrainBalancedActors(true)
+        .setRemindersStoragePartitions(7);
+
+    byte[] serialized = SERIALIZER.serialize(config);
+
+    String expected = "{\"entities\":[\"MyActor\"],"
+        + "\"actorIdleTimeout\":\"1h0m0s0ms\","
+        + "\"actorScanInterval\":\"0h0m30s0ms\","
+        + "\"drainOngoingCallTimeout\":\"0h1m0s0ms\","
+        + "\"drainBalancedActors\":true,"
+        + "\"remindersStoragePartitions\":7}";
+    Assertions.assertEquals(OBJECT_MAPPER.readTree(expected), OBJECT_MAPPER.readTree(serialized));
+  }
+
+  @Test
+  public void serializeActorRuntimeConfigWithActorTypeConfig() throws IOException {
+    ActorTypeConfig actorTypeConfig = new ActorTypeConfig()
+        .setActorTypeName("MyActor")
+        .setActorIdleTimeout(Duration.ofHours(2))
+        .setDrainBalancedActors(false)
+        .setRemindersStoragePartitions(3);
+    ActorRuntimeConfig config = new ActorRuntimeConfig()
+        .addRegisteredActorType("MyActor")
+        .addActorTypeConfig(actorTypeConfig);
+
+    byte[] serialized = SERIALIZER.serialize(config);
+
+    String expected = "{\"entities\":[\"MyActor\"],"
+        + "\"entitiesConfig\":[{"
+        + "\"entities\":[\"MyActor\"],"
+        + "\"actorIdleTimeout\":\"2h0m0s0ms\","
+        + "\"drainBalancedActors\":false,"
+        + "\"remindersStoragePartitions\":3}]}";
+    Assertions.assertEquals(OBJECT_MAPPER.readTree(expected), OBJECT_MAPPER.readTree(serialized));
+  }
+
+  @Test
+  public void customTypeRoundTripUsesDefaultSerialization() throws IOException {
+    CustomState original = new CustomState("myState", 42);
+
+    byte[] serialized = SERIALIZER.serialize(original);
+    CustomState recreated = SERIALIZER.deserialize(serialized, CustomState.class);
+
+    Assertions.assertEquals(original, recreated);
+  }
+}