Slither.io[a] (stylized as slither.io) is a multiplayer online video game available for iOS, Android, and web browsers, developed by Steve Howse.

The game grew in popularity following its promotion among several prominent YouTubers such as PewDiePie, and topped the App Store soon after its release.

[3] By pressing and holding the space bar, up arrow, or left/right mouse button, or double-tapping the touch screen on mobile, the player can use their boost, which causes the avatar to speed up using mass.

[6] The player may also choose to create their own skin, with a tool known as "Build a Slither", which shows the different colors that an avatar can be made of, that can be placed on the worm upon clicking.

Previously, in order to unlock custom skins in browser mode, players were forced to share the game on Twitter or Facebook using the external links found on the website.

[12] The only way Howse could make revenue was to display advertising in the app after the player's worm died; this option could be removed for US$3.99.

[13][14] In the weeks following the release, Howse worked on updates to stabilize the game and provide a better experience for players.

[15][16] In addition, he plans to add new features, such as a "friendly mode" that allows people to set up teams, and a way for the player to choose a server to play on.

[21] Brandt Ran, writing for Business Insider, said that "despite running into some technical hiccups—the game can lag heavily at times—I doubt Slither.io will be leaving my home screen anytime soon.

"[4] Harry Slater, writing for Pocket Gamer, defined the game as "interesting", the gameplay as a "compulsive experience", and the structure as simple and similar to Agar.io, although it did not have a large replay value.

[22] TechCrunch's Felicia Williams praised the designs, getting "pleasantly surprised" with the variety of skins for customization.

Scottie Rowland of Android Guys praised the gameplay and graphics but criticized the ads that pop up on the screen after the end of the game, calling them "extremely annoying", and finding the payment to remove them "a bit pricey".

[25] By July 2016, the browser version website was ranked by Alexa as the 250th most visited site worldwide, but then experienced a decline in popularity, dropping below 1,000 by October 2016 before remaining mostly constant at approximately 1,700 by January 2017.