"Slither.io" is a web massively multiplayer online game released in 2016, developed by Steve Howse. In the game, the player must control a creature that resembles a worm and capture various colored dots to increase its size. The ultimate goal of the game is to become the largest worm in the entire server. The game concept of "Slither.io" is similar to that of the popular web game "Agar.io" in 2015 and the classic arcade game "Snake" mode.

The web version and iOS version of the game were released on March 25, 2016, and it was popularized by many well-known YouTubers such as PewDiePie. Soon after the game was released, "Slither.io" topped the list of free software sales in the App Store in various regions, and the web version was ranked as the 250th most visited website in the world by Alexa in July 2016. The Android version was launched on March 27 of the same year.