diff --git a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
index e90ef5166988..d5578beef0ac 100644
--- a/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
+++ b/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java
@@ -7805,8 +7805,13 @@ protected void updateRequestBodyForArray(CodegenParameter codegenParameter, Sche
         }
     }
 
-    protected void updateRequestBodyForString(CodegenParameter codegenParameter, Schema schema, Set<String> imports, String bodyParameterName) {
-        updateRequestBodyForPrimitiveType(codegenParameter, schema, bodyParameterName, imports);
+    protected void updateRequestBodyForString(CodegenParameter codegenParameter, Schema schema, String name, Set<String> imports, String bodyParameterName) {
+        if (!StringUtils.isEmpty(name)) {
+            addBodyModelSchema(codegenParameter, name, schema, imports, bodyParameterName, false);
+        } else {
+            updateRequestBodyForPrimitiveType(codegenParameter, schema, bodyParameterName, imports);
+        }
+
         if (ModelUtils.isByteArraySchema(schema)) {
             codegenParameter.setIsString(false);
             codegenParameter.isByteArray = true;
@@ -8007,7 +8012,7 @@ public CodegenParameter fromRequestBody(RequestBody body, Set<String> imports, S
             // swagger v2 only, type file
             codegenParameter.isFile = true;
         } else if (ModelUtils.isStringSchema(schema)) {
-            updateRequestBodyForString(codegenParameter, schema, imports, bodyParameterName);
+            updateRequestBodyForString(codegenParameter, schema, name, imports, bodyParameterName);
         } else if (ModelUtils.isNumberSchema(schema)) {
             updateRequestBodyForPrimitiveType(codegenParameter, schema, bodyParameterName, imports);
             codegenParameter.isNumeric = Boolean.TRUE;
diff --git a/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java b/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
index 056a55a95d88..d31d6ab67a14 100644
--- a/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
+++ b/modules/openapi-generator/src/test/java/org/openapitools/codegen/DefaultCodegenTest.java
@@ -48,6 +48,7 @@
 import org.openapitools.codegen.utils.SemVer;
 import org.slf4j.LoggerFactory;
 import org.testng.Assert;
+import org.testng.annotations.DataProvider;
 import org.testng.annotations.Ignore;
 import org.testng.annotations.Test;
 
@@ -5044,4 +5045,40 @@ private List<String> getNames(List<CodegenProperty> props) {
         if (props == null) return null;
         return props.stream().map(v -> v.name).collect(Collectors.toList());
     }
-}
+
+    @Test
+    public void testRequestBodyWithStringEnumSchema() {
+        // Given
+        OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/issue_21407.yaml");
+        DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+        String path = "/v1/resource-class/send-using-schema";
+
+        // When
+        CodegenOperation codegenOperation = codegen.fromOperation(path, "POST", openAPI.getPaths().get(path).getPost(), null);
+
+        // Then
+        assertThat(codegenOperation.bodyParam).satisfies(bodyParam -> {
+            assertThat(bodyParam).isNotNull();
+            assertThat(bodyParam.getDataType()).isEqualTo("Letter");
+        });
+    }
+
+    @Test
+    public void testRequestBodyWithStringSchema() {
+        // Given
+        OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/issue_21407.yaml");
+        DefaultCodegen codegen = new DefaultCodegen();
+        codegen.setOpenAPI(openAPI);
+        String path = "/v1/resource-class/send-using-string";
+
+        // When
+        CodegenOperation codegenOperation = codegen.fromOperation(path, "POST", openAPI.getPaths().get(path).getPost(), null);
+
+        // Then
+        assertThat(codegenOperation.bodyParam).satisfies(bodyParam -> {
+            assertThat(bodyParam).isNotNull();
+            assertThat(bodyParam.getDataType()).isEqualTo("String");
+        });
+    }
+}
\ No newline at end of file
diff --git a/modules/openapi-generator/src/test/resources/3_0/issue_21407.yaml b/modules/openapi-generator/src/test/resources/3_0/issue_21407.yaml
new file mode 100644
index 000000000000..75eb086a6a0d
--- /dev/null
+++ b/modules/openapi-generator/src/test/resources/3_0/issue_21407.yaml
@@ -0,0 +1,48 @@
+openapi: 3.0.1
+info:
+  title: sample spec
+  description: "Sample spec"
+  version: 0.0.1
+tags:
+- name: ResourceClass
+paths:
+  /v1/resource-class/send-using-schema:
+    post:
+      tags:
+      - ResourceClass
+      description: Add @Operation annotation to provide a description
+      operationId: send-using-schema
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: "#/components/schemas/Letter"
+      responses:
+        "200":
+          description: OK - the request has succeeded.
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/Letter"
+  /v1/resource-class/send-using-string:
+    post:
+      tags:
+        - ResourceClass
+      description: Add @Operation annotation to provide a description
+      operationId: send-using-string
+      requestBody:
+        content:
+          application/json:
+            schema:
+              type: string
+      responses:
+        "204":
+          description: "No Content - the request has been successfully processed,\
+            \ but there is no additional content."
+components:
+  schemas:
+    Letter:
+      type: string
+      enum:
+      - A
+      - B
\ No newline at end of file