diff --git a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
index a0c3c4e5b7c..421b7782ebe 100644
--- a/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
+++ b/modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache
@@ -1081,6 +1081,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {
diff --git a/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
index 843d13b69a1..f2fca9d7ea3 100644
--- a/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java
@@ -871,6 +871,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {
diff --git a/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
index 69d6469c85a..ddd9a420bb6 100644
--- a/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java
@@ -972,6 +972,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {
diff --git a/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
index 7ef420e3c7c..93328801c2e 100644
--- a/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java
@@ -973,6 +973,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {
diff --git a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
index 7ef420e3c7c..93328801c2e 100644
--- a/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
+++ b/samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java
@@ -973,6 +973,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {
diff --git a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java
index 9edd0ae7bed..9daf2dcf886 100644
--- a/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java
+++ b/samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java
@@ -342,4 +342,22 @@ public class ApiClientTest {
     public void testNullHttpClient() {
         apiClient.setHttpClient(null);
     }
+
+    /**
+     * Tests the ApiClient serialize methods
+     */
+    @Test
+    public void testSerializeRequest() throws ApiException {
+        assertNotNull(apiClient.serialize("test", "text/plain"));
+        assertNotNull(apiClient.serialize("{}", "application/json"));
+    }
+
+    /**
+     * Tests the ApiClient serialize methods with unsupported content-type
+     * should raise ApiException
+     */
+    @Test(expected = ApiException.class)
+    public void testUnsupportedSerializeRequest() throws ApiException {
+        apiClient.serialize("test", "unsupported/type");
+    }
 }