diff --git a/.changeset/plain-views-rhyme.md b/.changeset/plain-views-rhyme.md
new file mode 100644
index 000000000..bbf19d189
--- /dev/null
+++ b/.changeset/plain-views-rhyme.md
@@ -0,0 +1,5 @@
+---
+"openapi-fetch": patch
+---
+
+fix omit serializing for already serialized data
diff --git a/packages/openapi-fetch/src/index.js b/packages/openapi-fetch/src/index.js
index be3b153a3..b54435a55 100644
--- a/packages/openapi-fetch/src/index.js
+++ b/packages/openapi-fetch/src/index.js
@@ -597,7 +597,7 @@ export function defaultPathSerializer(pathname, pathParams) {
  * @type {import("./index.js").defaultBodySerializer}
  */
 export function defaultBodySerializer(body, headers) {
-  if (body instanceof FormData) {
+  if (body instanceof FormData || typeof body === "string" || body instanceof URLSearchParams || body instanceof Blob) {
     return body;
   }
   if (headers) {
diff --git a/packages/openapi-fetch/test/common/request.test.ts b/packages/openapi-fetch/test/common/request.test.ts
index 7aa87e563..edb773e89 100644
--- a/packages/openapi-fetch/test/common/request.test.ts
+++ b/packages/openapi-fetch/test/common/request.test.ts
@@ -213,30 +213,32 @@ describe("request", () => {
     });
 
     test.each(BODY_ACCEPTING_METHODS)("`''` body (with body serializer) - %s", async (method) => {
+      const body = ''
       const bodySerializer = vi.fn((body) => `Serialized: ${JSON.stringify(body)}`);
       const { bodyUsed, bodyText } = await fireRequestAndGetBodyInformation({
         bodySerializer,
         method,
         fetchOptions: {
-          body: "",
+          body,
         },
       });
 
       expect(bodyUsed).toBe(true);
-      expect(bodyText).toBe('Serialized: ""');
+      expect(bodyText).toBe(`Serialized: ${body}`);
       expect(bodySerializer).toBeCalled();
     });
 
     test.each(BODY_ACCEPTING_METHODS)("`''` body (without body serializer) - %s", async (method) => {
+      const body = ''
       const { bodyUsed, bodyText } = await fireRequestAndGetBodyInformation({
         method,
         fetchOptions: {
-          body: "",
+          body,
         },
       });
 
       expect(bodyUsed).toBe(true);
-      expect(bodyText).toBe('""');
+      expect(bodyText).toBe(body);
     });
 
     test.each(BODY_ACCEPTING_METHODS)("`0` body (with body serializer) - %s", async (method) => {