Agar.io is a massively multiplayer action game developed by Matheus Valadares. Players control a cell in a map representing a petri dish; the goal is to gain as much mass as possible by swallowing smaller cells without being swallowed by bigger ones. The name Agar.io comes from the substanceagar, used to culture bacteria.

The game was initially advertised on 4chan on 28 April 2015 as a browser game. It quickly became popular largely due to word of mouth on social media, most notably YouTube and Twitch.tv. On 3 May 2015, Agar.io entered Steam Greenlight with Valadares planning to add features unavailable in the browserversion.^[7] It was approved by the community for listing on Steam. Miniclippublished an Android and iOS version on 8 July 2015.

Agar.io was released to a positive critical reception. Many praised the game's browser and mobile versions for its simplicity, competition, and mechanics, but some critics disliked its repetitiveness. The mobile versions received criticism for its controls, which were described as unwieldy. It was compared to Osmosand the cell stage of Spore.

Agar.io was quickly successful upon release; the agar.io website (for the browser version) was ranked by Alexa as one of the 1,000 most visited websites, and the mobile versions were downloaded more than ten million times during its first week.