diff --git a/src/index.ts b/src/index.ts
index 7a213ee34..015542956 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -134,15 +134,14 @@ const createHttpMethodFetcher =
   async <V extends Variables>(params: RequestVerbParams<V>) => {
     const { url, query, variables, operationName, fetch, fetchOptions, middleware } = params
 
-    const headers = { ...params.headers }
+    const headers = new Headers(params.headers as HeadersInit)
     let queryParams = ``
     let body = undefined
 
     if (method === `POST`) {
       body = createRequestBody(query, variables, operationName, fetchOptions.jsonSerializer)
-      if (typeof body === `string`) {
-        // @ts-expect-error todo
-        headers[`Content-Type`] = `application/json`
+      if (typeof body === `string` && !headers.has(`Content-Type`)) {
+        headers.set(`Content-Type`, `application/json`)
       }
     } else {
       // @ts-expect-error todo needs ADT for TS to understand the different states
diff --git a/src/types.ts b/src/types.ts
index b4aec590a..b4b7775a2 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -154,6 +154,8 @@ type RequestExtendedInit<V extends Variables = Variables> = RequestInit & {
   variables?: V
 }
 
+// TODO: Replace this type with the built-in `HeadersInit` type.
+// See: https://github.com/jasonkuhrt/graphql-request/issues/608
 export type GraphQLClientRequestHeaders = Headers | string[][] | Record<string, string>
 
 // prettier-ignore
diff --git a/tests/headers.test.ts b/tests/headers.test.ts
index a1754d82b..445068d3d 100644
--- a/tests/headers.test.ts
+++ b/tests/headers.test.ts
@@ -107,6 +107,24 @@ describe(`using class`, () => {
         expect(mock.requests[0]?.headers[`x-foo`]).toEqual(`new`)
       })
     })
+
+    describe(`allows content-type header to be overwritten`, () => {
+      test(`with request method`, async () => {
+        const headers = new Headers({ 'content-type': `text/plain` })
+        const client = new GraphQLClient(ctx.url, { headers })
+        const mock = ctx.res()
+        await client.request(`{ me { id } }`)
+        expect(mock.requests[0]?.headers[`content-type`]).toEqual(`text/plain`)
+      })
+
+      test(`with rawRequest method`, async () => {
+        const headers = new Headers({ 'content-type': `text/plain` })
+        const client = new GraphQLClient(ctx.url, { headers })
+        const mock = ctx.res()
+        await client.rawRequest(`{ me { id } }`)
+        expect(mock.requests[0]?.headers[`content-type`]).toEqual(`text/plain`)
+      })
+    })
   })
 })