diff --git a/src/core.ts b/src/core.ts
index e4af137..88f0eb5 100644
--- a/src/core.ts
+++ b/src/core.ts
@@ -92,7 +92,11 @@ export const core: Wretch = {
     let base = this.url(url).options({ method })
     // "Jsonify" the body if it is an object and if it is likely that the content type targets json.
     const contentType = extractContentType(base._options.headers)
-    const jsonify = typeof body === "object" && (!base._options.headers || !contentType || isLikelyJsonMime(contentType))
+    const isFormDataInstance = body => {
+      const formData = this._config.polyfill("FormData", false)
+      return formData !== null && body instanceof formData
+    }
+    const jsonify = typeof body === "object" && !isFormDataInstance(body) && (!base._options.headers || !contentType || isLikelyJsonMime(contentType))
     base =
       !body ? base :
         jsonify ? base.json(body, contentType) :
diff --git a/test/browser/wretch.spec.js b/test/browser/wretch.spec.js
index a7f9674..dcefa4d 100644
--- a/test/browser/wretch.spec.js
+++ b/test/browser/wretch.spec.js
@@ -134,6 +134,25 @@ describe("Wretch", function () {
     })
   })
 
+  it("should not Jasonify a FormData instance", async function () {
+    const FormData = wretch()._config.polyfill(
+      "FormData",
+      false
+    );
+
+    let formData = new FormData()
+    formData.append("hello", "world")
+    formData.append("duck", "Muscovy")
+
+    let decoded = await wretch(`${_URL}/formData/decode`)
+      .post(formData)
+      .json()
+    expect(decoded).toEqual({
+      hello: "world",
+      duck: "Muscovy",
+    })
+  })
+
   it("should perform OPTIONS and HEAD requests", async function () {
     const optsRes = await wretch(_URL + "/options").opts().res()
     const optsRes2 = await wretch(_URL + "/options").opts("").res()
diff --git a/test/deno/wretch_test.ts b/test/deno/wretch_test.ts
index b7715ee..42fa100 100644
--- a/test/deno/wretch_test.ts
+++ b/test/deno/wretch_test.ts
@@ -163,6 +163,25 @@ describe("Wretch", function () {
     })
   })
 
+  it("should not Jasonify a FormData instance", async function () {
+    const FormData = wretch()._config.polyfill(
+      "FormData",
+      false
+    );
+
+    let formData = new FormData()
+    formData.append("hello", "world")
+    formData.append("duck", "Muscovy")
+
+    let decoded = await wretch(`${_URL}/formData/decode`)
+      .post(formData)
+      .json()
+    assertEquals(decoded, {
+      hello: "world",
+      duck: "Muscovy",
+    })
+  })
+
   it("should perform OPTIONS and HEAD requests", async function () {
     const optsRes = await wretch(_URL + "/options").opts().res()
     const optsRes2 = await wretch(_URL + "/options").opts("").res()
diff --git a/test/node/wretch.spec.ts b/test/node/wretch.spec.ts
index ff11b8e..8a50c2c 100644
--- a/test/node/wretch.spec.ts
+++ b/test/node/wretch.spec.ts
@@ -251,6 +251,30 @@ describe("Wretch", function () {
     })
   })
 
+  it("should not Jasonify a FormData instance", async function () {
+    const FormData = wretch()._config.polyfill(
+      "FormData",
+      false
+    );
+
+    let formData = new FormData()
+    formData.append("hello", "world")
+    formData.append("duck", "Muscovy")
+    formData.append("duckImage", fs.createReadStream(duckImagePath))
+
+    let decoded = await wretch(`${_URL}/formData/decode`)
+      .post(formData)
+      .json()
+    expect(decoded).toEqual({
+      hello: "world",
+      duck: "Muscovy",
+      duckImage: {
+        data: duckImage,
+        type: "Buffer"
+      }
+    })
+  })
+
   it("should perform OPTIONS and HEAD requests", async function () {
     const optsRes = await wretch(_URL + "/options").opts().res()
     const optsRes2 = await wretch(_URL + "/options").opts("").res()