How to Lock the Bootloader on Samsung Galaxy S21 Ultra 5G

Verifying Bootloader Status

• 💡 First, ensure you are connected to a Wi-Fi network.
• 📌 Navigate to Settings > About phone > Software information and tap the Build number seven times to enable Developer options.
• 🔍 Within Developer options, confirm that OEM unlocking is enabled and grayed out, indicating the bootloader is unlocked.

Preparing to Lock the Bootloader

• ⚠️ Remember that locking or unlocking the bootloader will erase all your data; back up important files beforehand.
• 🔌 Power off your phone. Unplug it from any computer connection.
• ⚡ While holding Volume Up and Volume Down, plug the phone back into the computer to enter the blue download screen.

Locking the Bootloader

• 👆 Press and hold Volume Up on the blue screen.
• 🔒 The phone will prompt you to confirm locking the bootloader. Press Volume Up again to proceed.
• 🔄 The phone will reset. After the reset, you will need to set up the device again.

Finalizing Security Settings

• ⚙️ Once the phone is set up (you can do a quick setup without Wi-Fi initially), connect to Wi-Fi.
• 📱 Go back to Settings > About phone > Software information and tap Build number seven times.
• 🚫 In Developer options, disable OEM unlocking.
• 🔄 Press and hold the Power and Volume Down buttons simultaneously, then select restart to reboot your phone.
• ✅ Your bootloader will now be locked, restoring all security features and protecting your device.