Osu! is a and open source rhythm game originally created by Australian developer Dean Herbert in 2007. The gameplay is based on the Osu! Tatakae! Ouendan series and primarily involves clicking circles that appear on screen in time with music using a cursor or finger.

The game features four rulesets: osu!standard (click circles, sliders, and spinners), osu!taiko (drum-based notes inspired by Taiko no Tatsujin), osu!catch (catching falling fruit), and osu!mania (vertically scrolling key-based notes). Levels, called beatmaps, are created and uploaded by the community, providing a large and constantly expanding library. Players can compete on global leaderboards ranked by performance points or join real-time multiplayer rooms. The game supports keyboard, mouse, and graphics tablet input.