diff --git a/bin/utils/test_file_list.yaml b/bin/utils/test_file_list.yaml
index 86bd12d5d1bb..3ce25badeb3a 100644
--- a/bin/utils/test_file_list.yaml
+++ b/bin/utils/test_file_list.yaml
@@ -10,7 +10,7 @@
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ClientTest.java"
   sha256: 325fdd5d7e2c97790c0fb44f712ab7b2ba022d7e1a5b0056f47b07f342682b6d
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java"
-  sha256: ddd53d2f640aee7712299f1540779cdb508f0b9c1e96082191686e218eadb04c
+  sha256: 47293b138c51990bc80299837ed85c00463ccde9c4986fbe51d57246cf5eb0b7
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/api/PetApiTest.java"
   sha256: 8b1b8f2a2ad00ccb090873a94a5f73e328b98317d2ec715f53bd7a1accb2a023
 - filename: "samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/model/PetTest.java"
diff --git a/modules/openapi-generator/pom.xml b/modules/openapi-generator/pom.xml
index e3de9d3ee18d..27c2e7fe5fc8 100644
--- a/modules/openapi-generator/pom.xml
+++ b/modules/openapi-generator/pom.xml
@@ -303,6 +303,31 @@
             <version>2.10.1</version>
             <scope>test</scope>
         </dependency>
+        <!-- runtime dependencies of the generated okhttp-gson JSON factory, so that
+             JavaOkHttpGsonNullAdditionalPropertiesTest can compile and run generated
+             client sources (JSON.java + model) with the client's configured Gson.
+             javax.annotation.Nullable (jsr305) is intentionally NOT declared here:
+             a direct test-scope declaration would mediate the transitive
+             compile-scope jsr305 from guava down to test scope and break the
+             main-compile classpath.
+             okio must stay on a version whose Kotlin metadata is readable by the
+             kotlin-compiler-embeddable 1.6.21 on this module's test classpath
+             (KotlinTestUtils compiles generated Kotlin with the whole test
+             classpath as compiler input): okio 3.2.0 carries metadata 1.6.0,
+             while 3.4.0 carries 1.8.0 and broke KotlinTestUtilsTest,
+             KotlinMicroprofileClientCodegenTest and KotlinSpringServerCodegenTest. -->
+        <dependency>
+            <groupId>io.gsonfire</groupId>
+            <artifactId>gson-fire</artifactId>
+            <version>1.9.0</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.squareup.okio</groupId>
+            <artifactId>okio</artifactId>
+            <version>3.2.0</version>
+            <scope>test</scope>
+        </dependency>
         <dependency>
             <groupId>org.eclipse</groupId>
             <artifactId>yasson</artifactId>
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaOkHttpGsonNullAdditionalPropertiesTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaOkHttpGsonNullAdditionalPropertiesTest.java
new file mode 100644
index 000000000000..7c2d9a5ceef4
--- /dev/null
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/java/JavaOkHttpGsonNullAdditionalPropertiesTest.java
@@ -0,0 +1,126 @@
+/*
+ * Copyright 2026 OpenAPI-Generator Contributors (https://openapi-generator.tech)
+ *
+ * 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 org.openapitools.codegen.java;
+
+import com.google.gson.Gson;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import io.gsonfire.GsonFireBuilder;
+import okio.ByteString;
+import org.openapitools.codegen.DefaultGenerator;
+import org.openapitools.codegen.config.CodegenConfigurator;
+import org.testng.annotations.Test;
+
+import javax.tools.ToolProvider;
+import java.io.ByteArrayOutputStream;
+import java.io.File;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Path;
+import java.nio.file.Paths;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.Map;
+
+import static org.openapitools.codegen.TestUtils.newTempFolder;
+import static org.testng.Assert.*;
+
+public class JavaOkHttpGsonNullAdditionalPropertiesTest {
+
+    @Test
+    public void testNullAdditionalPropertiesRoundTrip() throws Exception {
+        Path output = newTempFolder();
+        CodegenConfigurator config = new CodegenConfigurator().setGeneratorName("java").setLibrary("okhttp-gson")
+                .setInputSpec("src/test/resources/3_0/java/okhttp-gson-null-additional-properties.yaml")
+                .setOutputDir(output.toString())
+                .addGlobalProperty("models", "").addGlobalProperty("supportingFiles", "")
+                .addGlobalProperty("modelDocs", "false").addGlobalProperty("modelTests", "false");
+        new DefaultGenerator().opts(config.toClientOptInput()).generate();
+
+        // Compile the generated client sources (the JSON factory and the model) against the real
+        // Gson runtime, mirroring the sample-side JSONTest setup: the model's CustomTypeAdapterFactory
+        // is registered by the generated JSON class, and JSON.java plus the generated model need the
+        // client's runtime dependencies (gson-fire, okio, jsr305 @Nullable) on the compile classpath.
+        Path generatedSource = output.resolve("src/main/java");
+        Path jsonSource = generatedSource.resolve("org/openapitools/client/JSON.java");
+        Path modelSource = generatedSource.resolve("org/openapitools/client/model/AdditionalPropertiesModel.java");
+        String classpath = Paths.get(Gson.class.getProtectionDomain().getCodeSource().getLocation().toURI())
+                + File.pathSeparator
+                + Paths.get(GsonFireBuilder.class.getProtectionDomain().getCodeSource().getLocation().toURI())
+                + File.pathSeparator
+                + Paths.get(ByteString.class.getProtectionDomain().getCodeSource().getLocation().toURI())
+                + File.pathSeparator
+                + Paths.get(javax.annotation.Nullable.class.getProtectionDomain().getCodeSource().getLocation().toURI())
+                + File.pathSeparator
+                + Paths.get(kotlin.jvm.internal.Intrinsics.class.getProtectionDomain().getCodeSource().getLocation().toURI());
+        ByteArrayOutputStream diagnostics = new ByteArrayOutputStream();
+        assertNotNull(ToolProvider.getSystemJavaCompiler());
+        assertEquals(ToolProvider.getSystemJavaCompiler().run(null, diagnostics, diagnostics,
+                "--release", "8", "-classpath", classpath, "-d", output.toString(),
+                jsonSource.toString(), modelSource.toString()),
+                0, diagnostics.toString(StandardCharsets.UTF_8));
+
+        try (URLClassLoader loader = new URLClassLoader(new URL[]{output.toUri().toURL()}, getClass().getClassLoader())) {
+            Class<?> jsonType = loader.loadClass("org.openapitools.client.JSON");
+            Class<?> modelType = loader.loadClass("org.openapitools.client.model.AdditionalPropertiesModel");
+            // the client's configured Gson (JSON.getGson), not a hand-built one: its static
+            // initializer registers the generated CustomTypeAdapterFactory for the model
+            Gson gson = (Gson) jsonType.getMethod("getGson").invoke(null);
+            // same pattern as the sample JSONTest: serializeNulls so named nulls are written
+            Gson gsonWithNulls = gson.newBuilder().serializeNulls().create();
+
+            // build the model with a null undeclared-property value
+            Object model = modelType.getConstructor().newInstance();
+            modelType.getMethod("setName", String.class).invoke(model, "x");
+            modelType.getMethod("putAdditionalProperty", String.class, Object.class).invoke(model, "unknownField", null);
+
+            // write: a null undeclared-property value used to throw
+            // IllegalStateException("Not a JSON Object: null") in the CustomTypeAdapterFactory;
+            // the ApiClient's writer omits named nulls, but must not throw
+            assertEquals(gson.toJson(model, modelType), "{\"name\":\"x\"}");
+
+            // serialize through the client's Gson and inspect the tree
+            String serialized = gsonWithNulls.toJson(model, modelType);
+            assertEquals(serialized, "{\"name\":\"x\",\"unknownField\":null}");
+            JsonObject obj = gsonWithNulls.toJsonTree(model, modelType).getAsJsonObject();
+            assertTrue(obj.get("unknownField").isJsonNull(), "A null additional property must be written as JSON null");
+            assertEquals(obj.get("name").getAsString(), "x");
+            assertNull(obj.get("additionalProperties"), "The raw additionalProperties field must be removed from the output");
+
+            // full round trip: deserialize the resulting JSON (not a hand-written string)
+            Object deserialized = gsonWithNulls.fromJson(serialized, modelType);
+            assertEquals(modelType.getMethod("getName").invoke(deserialized), "x");
+            Map<?, ?> props = (Map<?, ?>) modelType.getMethod("getAdditionalProperties").invoke(deserialized);
+            assertTrue(props.containsKey("unknownField"), "A JSON null undeclared property must be preserved by the round trip");
+            assertNull(props.get("unknownField"));
+
+            // serialize again: the round trip must be byte-stable
+            assertEquals(gsonWithNulls.toJson(deserialized, modelType), serialized, "Re-serialization must be byte-stable");
+            assertEquals(gson.toJson(deserialized, modelType), "{\"name\":\"x\"}");
+
+            // keep the existing behavior for non-null additional properties
+            modelType.getMethod("putAdditionalProperty", String.class, Object.class).invoke(model, "extra", "e");
+            modelType.getMethod("putAdditionalProperty", String.class, Object.class).invoke(model, "list", Arrays.asList("a", "b"));
+            modelType.getMethod("putAdditionalProperty", String.class, Object.class).invoke(model, "nested", Collections.singletonMap("k", "v"));
+            JsonElement tree = gsonWithNulls.toJsonTree(model, modelType);
+            obj = tree.getAsJsonObject();
+            assertEquals(obj.get("extra").getAsString(), "e");
+            assertTrue(obj.get("list").isJsonArray());
+            assertTrue(obj.get("nested").isJsonObject());
+        }
+    }
+}
diff --git a/modules/openapi-generator/src/test/resources/3_0/java/okhttp-gson-null-additional-properties.yaml b/modules/openapi-generator/src/test/resources/3_0/java/okhttp-gson-null-additional-properties.yaml
new file mode 100644
index 000000000000..581cbf5fce6e
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_0/java/okhttp-gson-null-additional-properties.yaml
@@ -0,0 +1,18 @@
+openapi: 3.0.3
+info: {title: t, version: '1'}
+paths:
+  /m:
+    get:
+      responses:
+        '200':
+          description: ok
+          content:
+            application/json:
+              schema: {$ref: '#/components/schemas/AdditionalPropertiesModel'}
+components:
+  schemas:
+    AdditionalPropertiesModel:
+      type: object
+      properties:
+        name: {type: string}
+      additionalProperties: true
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
index 86c22dc25e15..4da140e713ec 100644
--- a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/JSONTest.java
@@ -765,6 +765,39 @@ public void testAdditionalArrayProperties() throws IOException {
         assertEquals(z.toJson(), "{\"className\":\"zebra\",\"object_array\":[{\"id\":34.0,\"name\":\"just a tag\"}],\"empty_array\":[],\"array\":[\"1\",\"2\",\"3\"],\"new_array\":[\"1\",\"2\",\"3\"],\"new_empty_array\":[],\"new_object_array\":[{\"id\":34,\"name\":\"just a tag\"}]}");
     }
 
+    /**
+     * Test additional properties with a null value.
+     *
+     * Regression test for https://github.com/OpenAPITools/openapi-generator/issues/24393:
+     * the CustomTypeAdapterFactory used to throw IllegalStateException("Not a JSON Object: null")
+     * when serializing a model whose undeclared-property map contains a null value.
+     */
+    @Test
+    public void testAdditionalPropertiesNullValue() throws Exception {
+        // serialize: a null undeclared-property value no longer throws and is written as JSON null
+        AdditionalPropertiesClass model = new AdditionalPropertiesClass();
+        model.putAdditionalProperty("unknownField", null);
+        // Gson drops named nulls unless serializeNulls is enabled
+        Gson gsonWithNulls = json.getGson().newBuilder().serializeNulls().create();
+        JsonObject serialized = gsonWithNulls.toJsonTree(model).getAsJsonObject();
+        assertTrue(serialized.get("unknownField").isJsonNull());
+        // the ApiClient's Gson does not enable serializeNulls, so the named null is omitted from the text output
+        assertFalse(model.toJson().contains("unknownField"));
+
+        // deserialize: JSON null undeclared properties are stored as null-valued entries
+        AdditionalPropertiesClass deserialized = AdditionalPropertiesClass.fromJson("{\"map_property\":{\"key\":\"value\"},\"unknownField\":null}");
+        assertEquals(deserialized.getMapProperty(), Collections.singletonMap("key", "value"));
+        assertTrue(deserialized.getAdditionalProperties().containsKey("unknownField"));
+        assertNull(deserialized.getAdditionalProperties().get("unknownField"));
+
+        // serialize -> deserialize -> serialize round trip preserves the null entry
+        JsonObject roundTripped = gsonWithNulls.toJsonTree(deserialized).getAsJsonObject();
+        assertEquals(roundTripped.get("map_property").getAsJsonObject().get("key").getAsString(), "value");
+        assertTrue(roundTripped.get("unknownField").isJsonNull());
+        // text re-serialization does not throw
+        deserialized.toJson();
+    }
+
     /**
      * Validate a oneOf, anyOf schema with array sub-schema can be deserialized into the expected class.
      */