This article explains when and how to reinstall Windows Defender using the command line or the Settings app in Windows 11.

Windows Defender (also known as Microsoft Defender) doesn’t install on your computer like a regular program, so you can’t reinstall it like you can with regular software. Instead, follow these instructions to use a PowerShell command to get the job done. It can take anywhere from 5 seconds to a few minutes to complete. If you can open Settings normally, there’s an easier method after these steps.

Right-click the Start button and select Terminal (Admin).

If for some reason that doesn't work, use the search bar on your taskbar to search for PowerShell, then right-click on it to find the Run as administrator option.