Following the footsteps of the international variant of the Galaxy S3, the AT&T variant has now received the Android 6.0 marshmallow update. The update comes in the form of a CyanogenMod 13 ROM and remains an unofficial ROM. It has been build from the CM 13 sources available On github. The ROM has been ported to the US variant of the Galaxy S3 device. Network fixes comes with the initial build, and more are to be expected in future updates.

The Galaxy S3 seems to be a lucky device to have received the latest Android 6.0 Marshmallow update. The Marshmallow ROM runs, but several bugs prevail at the moment. Though the ROM may not look like a daily driver, it is a great deal for Android enthusiasts who want to test the latest Android software.

You will need TWRP to install the ROM. First flash the ROM and the listed Google apps from the download section. Remember, this is an Android 6.0 marshmallow ROM, hence lollipop Gapps won’t work.

Moreover, if you have multi-ROM support, then you can try it as a secondary ROM. Read the bugs section before you proceed to install the ROM. Also, visit the developer page for more information and updates from the download section.

Let’s begin with the tutorial.

Warning: Warranty may be void if you root or install custom ROM on your device. We at DroidViews won’t be liable for any damage to your device whatsoever. We are not responsible if you brick or damage your device. Reader’s discretion is advised.

 Bug Alert

  • This is a preview build, may not serve as a daily driver.
  • Network has been fixed and some other functions may not work.


Files for AT&T Samsung Galaxy S3:

  • CM 13 Marshmallow ROM — Download | File:
  • Marshmallow Gapps — Download | File:
  • Alternative Gapps — Download | File:


  • The listed custom ROM is compatible with AT&T Samsung Galaxy S3. Do not try it on any other device or variant.
  • Make sure you have a custom recovery like TWRP installed on your device. Install the latest TWRP build.
  • You must have device drivers installed on your PC so as to transfer the ROM and Google apps to your device’s storage.
  • You must fully charge your device or at least to 60%, as the process may take a while.

Backup your Apps and Data
Before you proceed, make sure you backup all your data. This process will wipe all the data from your device except for the internal storage. Hence, a full backup or nandroid backup is recommended. Also backup all your internal storage files just in case anything goes wrong. Take a nandroid backup from the custom recovery before flashing the ROM.

Install CyanogenMod 13 On AT&T Galaxy S3

  1. Download the Marshmallow CM 13 ROM of your choice from the download section.
  2. Now transfer the zip file into your device’s internal storage. Place the zip file where you can easily locate it.
  3. Now reboot into Custom Recovery (like TWRP). To boot into TWRP, first turn off your device and then press and hold volume down and power keys at the same time. If you see the TWRP logo, that means you have successfully booted into the recovery.
  4. It’s highly recommended that you perform a nandroid backup. To backup your ROM, tap on backup option in TWRP and select all the partitions including System, Data, Boot, Recovery, EFS and swipe right where appropriate, to backup.
  5. After performing the ROM backup, perform a full wipe. Return to the TWRP main menu and tap the wipe button and select Advanced Wipe. Then select Dalvik CacheSystemData, and Cache. Now swipe the confirmation button to wipe.
  6. Now go back to the TWRP main menu or home screen and tap on the install option. Navigate to the ROM file and select it.
  7. Swipe the confirmation button to proceed with the installation. The ROM will be installed.
  8. Now return to home in the TWRP and tap on the install option. Navigate to the Google apps zip file and select it.
  9. Swipe the confirmation button to proceed with the installation.
  10. Once installed, reboot to system.

If you see the new boot logo, that means you have successfully installed the ROM on your device. If you come across any boot loop or face system errors, try a factory reset from the recovery or you can always restore your previous ROM from the backup. If you lose root access on your new ROM, get to this page for latest SuperSU build.

Also See:

How to Install AOSP 6.0 on Galaxy S3 i9300

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