Agar.io is an online multiplayer game about growing cells and consuming others in a closed arena. Multiple players on several servers fight each other to grow the largest cell. The cell can be moved in any direction and needs to eat pellets that make the cell's mass grow. Other players do the same and with growth movement becomes slower. A player's cell can consume another if it is distinctly larger in size, absorbing it and ending the game for the other player. It is possible to restart right away, but with the original small size. The arena contains viruses that can make a cell explode, eating away from the cell and splitting it up.
Next to movement two more actions are available. It is possible to split the cell in half, launching it forward (at half the original size), in an attempt to absorb another, faster cell by surprise. The second action is ejecting mass. It can be used to gain speed quickly, as bait, to throw viruses through enemy cells, to recover after a virus explosion or to form a temporary alliance with another player by providing a gift. There are no means to communicate with other players in the game. Cells are assigned a random colour and the player can choose a name. There are also several pre-defined skins in the game, referencing countries, website or world leaders. These skins can be used by entering the correct name as the nickname.
The main game mode is a free-for-all mode, but for the browser version there are also three other ones: experimental, party and team.