diff --git a/ts/services/backups/api.ts b/ts/services/backups/api.ts
index ed1ef097537e..d0f2a635f954 100644
--- a/ts/services/backups/api.ts
+++ b/ts/services/backups/api.ts
@@ -1,6 +1,8 @@
 // Copyright 2024 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
+import type { Readable } from 'stream';
+
 import { strictAssert } from '../../util/assert';
 import type {
   WebAPIType,
@@ -53,10 +55,11 @@ export class BackupAPI {
     return (await this.getInfo()).backupName;
   }
 
-  public async getUploadForm(): Promise<GetBackupUploadFormResponseType> {
-    return this.server.getBackupUploadForm(
-      await this.credentials.getHeadersForToday()
-    );
+  public async upload(stream: Readable): Promise<string> {
+    return this.server.uploadBackup({
+      headers: await this.credentials.getHeadersForToday(),
+      stream,
+    });
   }
 
   public async getMediaUploadForm(): Promise<GetBackupUploadFormResponseType> {
diff --git a/ts/services/backups/index.ts b/ts/services/backups/index.ts
index ecc8d770f7d4..9d1dcd641307 100644
--- a/ts/services/backups/index.ts
+++ b/ts/services/backups/index.ts
@@ -90,6 +90,12 @@ export class BackupsService {
     }
   }
 
+  public async upload(): Promise<void> {
+    const pipe = new PassThrough();
+
+    await Promise.all([this.api.upload(pipe), this.exportBackup(pipe)]);
+  }
+
   // Test harness
   public async exportBackupData(): Promise<Uint8Array> {
     const sink = new PassThrough();
diff --git a/ts/textsecure/WebAPI.ts b/ts/textsecure/WebAPI.ts
index fd3cd7d8ac5d..bd49dedc1677 100644
--- a/ts/textsecure/WebAPI.ts
+++ b/ts/textsecure/WebAPI.ts
@@ -143,6 +143,7 @@ function _validateResponse(response: any, schema: any) {
 
 const FIVE_MINUTES = 5 * durations.MINUTE;
 const GET_ATTACHMENT_CHUNK_TIMEOUT = 10 * durations.SECOND;
+const BACKUP_CDN_VERSION = 3;
 
 type AgentCacheType = {
   [name: string]: {
@@ -171,7 +172,7 @@ type PromiseAjaxOptionsType = {
   basicAuth?: string;
   certificateAuthority?: string;
   contentType?: string;
-  data?: Uint8Array | string;
+  data?: Uint8Array | Readable | string;
   disableRetries?: boolean;
   disableSessionResumption?: boolean;
   headers?: HeaderListType;
@@ -1013,6 +1014,11 @@ export type SetBackupSignatureKeyOptionsType = Readonly<{
   backupIdPublicKey: Uint8Array;
 }>;
 
+export type UploadBackupOptionsType = Readonly<{
+  headers: BackupPresentationHeadersType;
+  stream: Readable;
+}>;
+
 export type BackupMediaItemType = Readonly<{
   sourceAttachment: Readonly<{
     cdn: number;
@@ -1354,6 +1360,7 @@ export type WebAPIType = {
     uploadAvatarRequestHeaders: UploadAvatarHeadersType,
     avatarData: Uint8Array
   ) => Promise<string>;
+  uploadBackup: (options: UploadBackupOptionsType) => Promise<string>;
   uploadGroupAvatar: (
     avatarData: Uint8Array,
     options: GroupCredentialsType
@@ -1720,6 +1727,7 @@ export function initialize({
       unregisterRequestHandler,
       updateDeviceName,
       uploadAvatar,
+      uploadBackup,
       uploadGroupAvatar,
       whoami,
     };
@@ -2733,6 +2741,93 @@ export function initialize({
       return getBackupUploadFormResponseSchema.parse(res);
     }
 
+    async function uploadBackup({ headers, stream }: UploadBackupOptionsType) {
+      const {
+        signedUploadLocation,
+        headers: uploadHeaders,
+        cdn,
+        key,
+      } = await getBackupUploadForm(headers);
+
+      strictAssert(
+        cdn === BACKUP_CDN_VERSION,
+        'uploadBackup: unexpected cdn version'
+      );
+
+      let size = 0n;
+      stream.pause();
+      stream.on('data', chunk => {
+        size += BigInt(chunk.length);
+      });
+
+      const uploadOptions = {
+        certificateAuthority,
+        proxyUrl,
+        timeout: 0,
+        type: 'POST' as const,
+        version,
+        headers: {
+          ...uploadHeaders,
+          'Tus-Resumable': '1.0.0',
+          'Content-Type': 'application/offset+octet-stream',
+          'Upload-Defer-Length': '1',
+        },
+        redactUrl: () => {
+          const tmp = new URL(signedUploadLocation);
+          tmp.search = '';
+          tmp.pathname = '';
+          return `${tmp}[REDACTED]`;
+        },
+        data: stream,
+        responseType: 'byteswithdetails' as const,
+      };
+
+      let response: Response;
+      try {
+        ({ response } = await _outerAjax(signedUploadLocation, uploadOptions));
+      } catch (e) {
+        // Another upload in progress, getting 409 should have aborted it.
+        if (e instanceof HTTPError && e.code === 409) {
+          log.warn('uploadBackup: aborting previous unfinished upload');
+          ({ response } = await _outerAjax(
+            signedUploadLocation,
+            uploadOptions
+          ));
+        } else {
+          throw e;
+        }
+      }
+
+      const uploadLocation = response.headers.get('location');
+      strictAssert(uploadLocation, 'backup response header has no location');
+
+      // Finish the upload by sending a PATCH with the stream length
+
+      // This is going to the CDN, not the service, so we use _outerAjax
+      await _outerAjax(uploadLocation, {
+        certificateAuthority,
+        proxyUrl,
+        timeout: 0,
+        type: 'PATCH',
+        version,
+        headers: {
+          ...uploadHeaders,
+          'Tus-Resumable': '1.0.0',
+          'Content-Type': 'application/offset+octet-stream',
+          'Upload-Offset': String(size),
+          'Upload-Length': String(size),
+        },
+        redactUrl: () => {
+          const tmp = new URL(uploadLocation);
+          tmp.search = '';
+          tmp.pathname = '';
+          return `${tmp}[REDACTED]`;
+        },
+      });
+
+      return key;
+    }
+
     async function refreshBackup(headers: BackupPresentationHeadersType) {
       await _ajax({
         call: 'backup',