A UNITY3D file is a 3D game archive used by a web browser with the Unity Web Player plugin installed to play a 3D game online that was developed by Unity. It contains assets, such as models, textures, animations, and sounds, and other game data that was built in Unity from a .UNITY scene file.

The assets within a UNITY3D file are stored in a proprietary, closed format to protect the game. The archive is only meant to be accessed using the Unity Web Player plugin in order to play the compiled game. To open a UNITY3D file, download and install Unity Web Player, then open the UNITY3D file in a web browser, such as Mozilla Firefox or Google Chrome. You may have to right-click the UNITY3D file and choose the web browser through the "Open with" option. NOTE: Unity Web Player has been discontinued and is no longer developed.