KoalaSync is a free, open source browser extension available for Chrome and Firefox that enables synchronized video watching across multiple users. It supports popular streaming platforms such as Netflix, YouTube, and Twitch, as well as any website using standard HTML5 video tags. The extension does not require an account, does not track user activity, and displays no advertisements. Synchronization is achieved by sharing a generated invite link; participants join a room with a single click and their video playheads are automatically linked. Only synchronization signals leave the browser; the video content itself never transmits through the service. Key features include instant room creation and one-click joining, a built-in audio compressor that levels volume differences locally in the browser, and broad compatibility with HTML5 video players. The tool is designed for real time sync, allowing users to pause, play, and seek together while communicating through an external voice call application such as Discord or Zoom. Typical use cases include remote movie nights with friends, collaborative viewing of online tutorials or lectures for remote learning, and shared date nights for long distance relationships. No IP addresses or passwords need to be exchanged, as the invite link handles access. Technically, KoalaSync works on websites that permit script injections into standard HTML5 video tags. It may face limitations on sites with strict Content Security Policies, DRM copy protection, or heavily obfuscated player wrappers such as complex shadow DOMs. The extension is open source and operates on a privacy first principle. A live demo is available on the project website at https://sync.koalastuff.net, and a comparison with similar services like Teleparty is provided on that site.