diff --git a/core/src/main/java/com/google/adk/agents/RunConfig.java b/core/src/main/java/com/google/adk/agents/RunConfig.java
index 2f8e417d..308169e3 100644
--- a/core/src/main/java/com/google/adk/agents/RunConfig.java
+++ b/core/src/main/java/com/google/adk/agents/RunConfig.java
@@ -70,6 +70,8 @@ public enum ToolExecutionMode {
 
   public abstract int maxLlmCalls();
 
+  public abstract boolean autoCreateSession();
+
   public abstract Builder toBuilder();
 
   public static Builder builder() {
@@ -78,7 +80,8 @@ public static Builder builder() {
         .setResponseModalities(ImmutableList.of())
         .setStreamingMode(StreamingMode.NONE)
         .setToolExecutionMode(ToolExecutionMode.NONE)
-        .setMaxLlmCalls(500);
+        .setMaxLlmCalls(500)
+        .setAutoCreateSession(false);
   }
 
   public static Builder builder(RunConfig runConfig) {
@@ -90,7 +93,8 @@ public static Builder builder(RunConfig runConfig) {
         .setResponseModalities(runConfig.responseModalities())
         .setSpeechConfig(runConfig.speechConfig())
         .setOutputAudioTranscription(runConfig.outputAudioTranscription())
-        .setInputAudioTranscription(runConfig.inputAudioTranscription());
+        .setInputAudioTranscription(runConfig.inputAudioTranscription())
+        .setAutoCreateSession(runConfig.autoCreateSession());
   }
 
   /** Builder for {@link RunConfig}. */
@@ -123,6 +127,9 @@ public abstract Builder setInputAudioTranscription(
     @CanIgnoreReturnValue
     public abstract Builder setMaxLlmCalls(int maxLlmCalls);
 
+    @CanIgnoreReturnValue
+    public abstract Builder setAutoCreateSession(boolean autoCreateSession);
+
     abstract RunConfig autoBuild();
 
     public RunConfig build() {
diff --git a/core/src/main/java/com/google/adk/runner/Runner.java b/core/src/main/java/com/google/adk/runner/Runner.java
index e047d1e6..c5c18cdf 100644
--- a/core/src/main/java/com/google/adk/runner/Runner.java
+++ b/core/src/main/java/com/google/adk/runner/Runner.java
@@ -378,9 +378,15 @@ public Flowable<Event> runAsync(
         this.sessionService.getSession(appName, userId, sessionId, Optional.empty());
     return maybeSession
         .switchIfEmpty(
-            Single.error(
-                new IllegalArgumentException(
-                    String.format("Session not found: %s for user %s", sessionId, userId))))
+            Single.defer(
+                () -> {
+                  if (runConfig.autoCreateSession()) {
+                    return this.sessionService.createSession(appName, userId, null, sessionId);
+                  }
+                  return Single.error(
+                      new IllegalArgumentException(
+                          String.format("Session not found: %s for user %s", sessionId, userId)));
+                }))
         .flatMapPublisher(session -> this.runAsync(session, newMessage, runConfig, stateDelta));
   }
 
@@ -668,15 +674,17 @@ public Flowable<Event> runLive(
       String userId, String sessionId, LiveRequestQueue liveRequestQueue, RunConfig runConfig) {
     return this.sessionService
         .getSession(appName, userId, sessionId, Optional.empty())
-        .flatMapPublisher(
-            session -> {
-              if (session == null) {
-                return Flowable.error(
-                    new IllegalArgumentException(
-                        String.format("Session not found: %s for user %s", sessionId, userId)));
-              }
-              return this.runLive(session, liveRequestQueue, runConfig);
-            });
+        .switchIfEmpty(
+            Single.defer(
+                () -> {
+                  if (runConfig.autoCreateSession()) {
+                    return this.sessionService.createSession(appName, userId, null, sessionId);
+                  }
+                  return Single.error(
+                      new IllegalArgumentException(
+                          String.format("Session not found: %s for user %s", sessionId, userId)));
+                }))
+        .flatMapPublisher(session -> this.runLive(session, liveRequestQueue, runConfig));
   }
 
   /**
diff --git a/core/src/test/java/com/google/adk/runner/RunnerTest.java b/core/src/test/java/com/google/adk/runner/RunnerTest.java
index 994acca2..f1a01100 100644
--- a/core/src/test/java/com/google/adk/runner/RunnerTest.java
+++ b/core/src/test/java/com/google/adk/runner/RunnerTest.java
@@ -59,6 +59,7 @@
 import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
+import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import org.junit.After;
 import org.junit.Before;
@@ -76,10 +77,8 @@ public final class RunnerTest {
   private final Content pluginContent = createContent("from plugin");
   private final TestLlm testLlm = createTestLlm(createLlmResponse(createContent("from llm")));
   private final LlmAgent agent = createTestAgentBuilder(testLlm).build();
-  private final Runner runner =
-      Runner.builder().agent(agent).appName("test").plugins(ImmutableList.of(plugin)).build();
-  private final Session session =
-      runner.sessionService().createSession("test", "user").blockingGet();
+  private Runner runner;
+  private Session session;
   private Tracer originalTracer;
 
   private final FailingEchoTool failingEchoTool = new FailingEchoTool();
@@ -114,6 +113,9 @@ private BasePlugin mockPlugin(String name) {
   public void setUp() {
     this.originalTracer = Telemetry.getTracer();
     Telemetry.setTracerForTesting(openTelemetryRule.getOpenTelemetry().getTracer("RunnerTest"));
+    this.runner =
+        Runner.builder().agent(agent).appName("test").plugins(ImmutableList.of(plugin)).build();
+    this.session = runner.sessionService().createSession("test", "user").blockingGet();
   }
 
   @After
@@ -832,4 +834,70 @@ public void resumabilityConfig_isNotResumable_isFalseInInvocationContext() {
         runner.runAsync("user", session.id(), createContent("from user")).toList().blockingGet();
     assertThat(contextCaptor.getValue().isResumable()).isFalse();
   }
+
+  @Test
+  public void runAsync_withoutSessionAndAutoCreateSessionTrue_createsSession() {
+    RunConfig runConfig = RunConfig.builder().setAutoCreateSession(true).build();
+    String newSessionId = UUID.randomUUID().toString();
+
+    var events =
+        runner
+            .runAsync("user", newSessionId, createContent("from user"), runConfig)
+            .toList()
+            .blockingGet();
+
+    assertThat(simplifyEvents(events)).containsExactly("test agent: from llm");
+    assertThat(
+            runner
+                .sessionService()
+                .getSession("test", "user", newSessionId, Optional.empty())
+                .blockingGet())
+        .isNotNull();
+  }
+
+  @Test
+  public void runAsync_withoutSessionAndAutoCreateSessionFalse_throwsException() {
+    RunConfig runConfig = RunConfig.builder().setAutoCreateSession(false).build();
+    String newSessionId = UUID.randomUUID().toString();
+
+    runner
+        .runAsync("user", newSessionId, createContent("from user"), runConfig)
+        .test()
+        .assertError(IllegalArgumentException.class);
+  }
+
+  @Test
+  public void runLive_withoutSessionAndAutoCreateSessionTrue_createsSession() throws Exception {
+    RunConfig runConfig = RunConfig.builder().setAutoCreateSession(true).build();
+    String newSessionId = UUID.randomUUID().toString();
+    LiveRequestQueue liveRequestQueue = new LiveRequestQueue();
+
+    TestSubscriber<Event> testSubscriber =
+        runner.runLive("user", newSessionId, liveRequestQueue, runConfig).test();
+
+    liveRequestQueue.content(createContent("from user"));
+    liveRequestQueue.close();
+
+    testSubscriber.await();
+    testSubscriber.assertComplete();
+    assertThat(simplifyEvents(testSubscriber.values())).containsExactly("test agent: from llm");
+    assertThat(
+            runner
+                .sessionService()
+                .getSession("test", "user", newSessionId, Optional.empty())
+                .blockingGet())
+        .isNotNull();
+  }
+
+  @Test
+  public void runLive_withoutSessionAndAutoCreateSessionFalse_throwsException() {
+    RunConfig runConfig = RunConfig.builder().setAutoCreateSession(false).build();
+    String newSessionId = UUID.randomUUID().toString();
+    LiveRequestQueue liveRequestQueue = new LiveRequestQueue();
+
+    runner
+        .runLive("user", newSessionId, liveRequestQueue, runConfig)
+        .test()
+        .assertError(IllegalArgumentException.class);
+  }
 }