YouTube Watched Marker is a Chrome extension developed by Giray Bal, available for free in the Browsers category under Add-ons & Tools. This lightweight extension provides a simple solution to mark the YouTube videos you have already watched while browsing YouTube.com.

With YouTube Watched Marker, users can conveniently hide or show watched videos with just a single click. Additionally, the extension offers the functionality to create a backup of all watched videos, allowing users to download the list as a .txt file. The watchlist is stored in the Chrome cache, enabling users to periodically back up their list and restore it if the Chrome cache is cleared.