diff --git a/packages/@uppy/aws-s3/src/HTTPCommunicationQueue.ts b/packages/@uppy/aws-s3/src/HTTPCommunicationQueue.ts
index c015633ad9..d349fc6823 100644
--- a/packages/@uppy/aws-s3/src/HTTPCommunicationQueue.ts
+++ b/packages/@uppy/aws-s3/src/HTTPCommunicationQueue.ts
@@ -253,7 +253,7 @@ export class HTTPCommunicationQueue<M extends Meta, B extends Body> {
     }).abortOn(signal)
 
     let body: FormData | Blob
-    const data = chunk.getData()
+    const data = await chunk.getData()
     if (method.toUpperCase() === 'POST') {
       const formData = new FormData()
       Object.entries(fields!).forEach(([key, value]) =>
@@ -387,7 +387,7 @@ export class HTTPCommunicationQueue<M extends Meta, B extends Body> {
 
     for (;;) {
       throwIfAborted(signal)
-      const chunkData = chunk.getData()
+      const chunkData = await chunk.getData()
       const { onProgress, onComplete } = chunk
       let signature
 
diff --git a/packages/@uppy/aws-s3/src/MultipartUploader.ts b/packages/@uppy/aws-s3/src/MultipartUploader.ts
index 6f5397a7fd..5dad800c0e 100644
--- a/packages/@uppy/aws-s3/src/MultipartUploader.ts
+++ b/packages/@uppy/aws-s3/src/MultipartUploader.ts
@@ -7,6 +7,7 @@ const MB = 1024 * 1024
 
 interface MultipartUploaderOptions<M extends Meta, B extends Body> {
   getChunkSize?: (file: { size: number }) => number
+  getChunkData?: (start: number, end: number, opts: { chunkSize: number }) => Promise<Blob>
   onProgress?: (bytesUploaded: number, bytesTotal: number) => void
   onPartComplete?: (part: { PartNumber: number; ETag: string }) => void
   shouldUseMultipart?: boolean | ((file: UppyFile<M, B>) => boolean)
@@ -33,7 +34,7 @@ const defaultOptions = {
 } satisfies Partial<MultipartUploaderOptions<any, any>>
 
 export interface Chunk {
-  getData: () => Blob
+  getData: () => Promise<Blob>
   onProgress: (ev: ProgressEvent) => void
   onComplete: (etag: string) => void
   shouldUseMultipart: boolean
@@ -146,9 +147,10 @@ class MultipartUploader<M extends Meta, B extends Body> {
         const end = Math.min(fileSize, offset + chunkSize)
 
         // Defer data fetching/slicing until we actually need the data, because it's slow if we have a lot of files
-        const getData = () => {
+        const getData = async () => {
           const i2 = offset
-          return this.#data.slice(i2, end)
+          return this.options.getChunkData ? this.options.getChunkData(i2, end, { chunkSize }) :
+            this.#data.slice(i2, end)
         }
 
         this.#chunks[j] = {
@@ -171,7 +173,7 @@ class MultipartUploader<M extends Meta, B extends Body> {
     } else {
       this.#chunks = [
         {
-          getData: () => this.#data,
+          getData: async () => this.#data,
           onProgress: this.#onPartProgress(0),
           onComplete: this.#onPartComplete(0),
           shouldUseMultipart,