It depends on which “Defender” you’re talking about. If you’re using Microsoft Defender Antivirus as your primary antivirus in Windows Security and decide to install a third-party antivirus, Microsoft Defender AV will switch to passive mode letting the other antivirus take over. It will continue to receive updates but won’t scan your device for malware or other threats.

However, if you have a paid subscription to Microsoft Defender, it’s different as it’s an endpoint solution. It will run alongside any compatible third-party antivirus, such as Malwarebytes or Bitdefender, and will also work alongside their endpoint services.