Osu! (stylized as osu!) is a free-to-play rhythm game primarily developed, published, and created by Dean "peppy" Herbert. Inspired by iNiS' rhythm game Osu! Tatakae! Ouendan, it was written in C# on the .NET Framework, and was released for Microsoft Windows on 16 September 2007. The game has throughout the years been ported to macOS, Linux, Android and iOS.

Playing this game is mainly "clicking circles". For example, the main mode of the game (known as std mode) is to click the circle consistently appearing on the screen.