diff --git a/pulsar-client-v5/src/main/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapter.java b/pulsar-client-v5/src/main/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapter.java
index 6b4c6ba38d3b5..bb51d2f9107e6 100644
--- a/pulsar-client-v5/src/main/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapter.java
+++ b/pulsar-client-v5/src/main/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapter.java
@@ -18,8 +18,12 @@
  */
 package org.apache.pulsar.client.impl.v5;
 
+import java.security.PrivateKey;
+import java.security.cert.Certificate;
 import java.util.Map;
+import java.util.Set;
 import java.util.function.Supplier;
+import org.apache.pulsar.client.api.AuthenticationDataProvider;
 import org.apache.pulsar.client.api.v5.PulsarClientException;
 import org.apache.pulsar.client.api.v5.auth.Authentication;
 import org.apache.pulsar.client.api.v5.auth.AuthenticationData;
@@ -107,8 +111,22 @@ public String authMethodName() {
 
         @Override
         public AuthenticationData authData() throws PulsarClientException {
-            // TODO: Implement AuthenticationData adapter when needed
-            return null;
+            try {
+                AuthenticationDataProvider v4Data = v4Auth.getAuthData();
+                return v4Data == null ? null : new V5AuthDataWrapper(v4Data);
+            } catch (org.apache.pulsar.client.api.PulsarClientException e) {
+                throw new PulsarClientException(e.getMessage(), e);
+            }
+        }
+
+        @Override
+        public AuthenticationData authData(String brokerHostName) throws PulsarClientException {
+            try {
+                AuthenticationDataProvider v4Data = v4Auth.getAuthData(brokerHostName);
+                return v4Data == null ? null : new V5AuthDataWrapper(v4Data);
+            } catch (org.apache.pulsar.client.api.PulsarClientException e) {
+                throw new PulsarClientException(e.getMessage(), e);
+            }
         }
 
         @Override
@@ -129,4 +147,56 @@ public void close() {
             }
         }
     }
+
+    /**
+     * Bridges a v4 {@link AuthenticationDataProvider} into the V5 {@link AuthenticationData}
+     * surface. The two interfaces are intentionally aligned so this is a 1:1 delegation.
+     */
+    private static final class V5AuthDataWrapper implements AuthenticationData {
+        private final AuthenticationDataProvider v4Data;
+
+        V5AuthDataWrapper(AuthenticationDataProvider v4Data) {
+            this.v4Data = v4Data;
+        }
+
+        @Override
+        public boolean hasDataForHttp() {
+            return v4Data.hasDataForHttp();
+        }
+
+        @Override
+        public Set<Map.Entry<String, String>> getHttpHeaders() {
+            try {
+                Set<Map.Entry<String, String>> headers = v4Data.getHttpHeaders();
+                return headers != null ? headers : Set.of();
+            } catch (Exception e) {
+                throw new RuntimeException("Failed to obtain HTTP auth headers", e);
+            }
+        }
+
+        @Override
+        public boolean hasDataForTls() {
+            return v4Data.hasDataForTls();
+        }
+
+        @Override
+        public Certificate[] getTlsCertificates() {
+            return v4Data.getTlsCertificates();
+        }
+
+        @Override
+        public PrivateKey getTlsPrivateKey() {
+            return v4Data.getTlsPrivateKey();
+        }
+
+        @Override
+        public boolean hasDataFromCommand() {
+            return v4Data.hasDataFromCommand();
+        }
+
+        @Override
+        public String getCommandData() {
+            return v4Data.getCommandData();
+        }
+    }
 }
diff --git a/pulsar-client-v5/src/test/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapterTest.java b/pulsar-client-v5/src/test/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapterTest.java
new file mode 100644
index 0000000000000..283cff17b157d
--- /dev/null
+++ b/pulsar-client-v5/src/test/java/org/apache/pulsar/client/impl/v5/AuthenticationAdapterTest.java
@@ -0,0 +1,62 @@
+/*
+ * 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.pulsar.client.impl.v5;
+
+import static org.testng.Assert.assertEquals;
+import static org.testng.Assert.assertNotNull;
+import static org.testng.Assert.assertTrue;
+import org.apache.pulsar.client.api.v5.auth.Authentication;
+import org.apache.pulsar.client.api.v5.auth.AuthenticationData;
+import org.testng.annotations.Test;
+
+/**
+ * Verifies that {@code AuthenticationAdapter}'s V5 wrapper bridges through to the
+ * v4 {@code AuthenticationDataProvider} on every dimension exposed by the V5
+ * {@link AuthenticationData} surface — previously {@code authData()} was a TODO
+ * stub returning {@code null}, which silently broke any V5 caller that asked the
+ * wrapped auth for credentials.
+ */
+public class AuthenticationAdapterTest {
+
+    @Test
+    public void testTokenAuthExposesCommandData() throws Exception {
+        Authentication v5Auth = AuthenticationAdapter.token("my-jwt");
+
+        AuthenticationData data = v5Auth.authData();
+        assertNotNull(data, "authData() must not return null after the wiring fix");
+        assertTrue(data.hasDataFromCommand(),
+                "token auth should expose its credential via the binary-protocol command path");
+        assertEquals(data.getCommandData(), "my-jwt");
+    }
+
+    @Test
+    public void testAuthMethodNamePassesThrough() throws Exception {
+        Authentication v5Auth = AuthenticationAdapter.token("any-token");
+        assertEquals(v5Auth.authMethodName(), "token");
+    }
+
+    @Test
+    public void testAuthDataPerBrokerHostDelegates() throws Exception {
+        Authentication v5Auth = AuthenticationAdapter.token("per-host-token");
+
+        AuthenticationData data = v5Auth.authData("broker-1.example.com");
+        assertNotNull(data, "authData(brokerHost) must not return null");
+        assertEquals(data.getCommandData(), "per-host-token");
+    }
+}