January 31, 2020
March 22, 2020
|Latest release||2.3.0 / April 18, 2020|
|Platform(s)||Web (Native) |
Windows, Linux, macOS
|Next pieces||5 (default)|
|Playfield dimensions||10 × 20|
|Rotation system||SRS, SRS-X, ARS, NRS, ASC|
Tetr.io is a free-to-play online multiplayer and single player fan game developed by osk.
Tetr.io offers multiplayer and solo play. There are currently three single player modes and two multiplayer modes.
In multiplayer, the player can play matches against other players. A player can choose to play in the game's Quick Play room where players can play a match with default settings. The Custom Play option allows for creation of custom rooms, with the ability to change settings for the room such as spin bonuses, player limit, the amount of next pieces shown, and others. The player can view public rooms in the Room Listing menu. Each multiplayer game with 3 or more players plays similarly to Tetris 99 along with the targeting options included. A margin time system is also in the game, in which the amount of garbage sent is increased after every once in a while.
Tetra League is the ranked mode in Tetr.io. As of its public release, players must be registered and their level must be at least Lv. 10 to enter, and anonymous players cannot play this mode. When the mode was first released, INFDEV testers and supporters were only eligible for the mode. Each game of Tetra League is a 1v1 match, with a FT3, FT5, or FT7 format based on one of the players' rank. The rating system is called TR, which is based on Glicko 2. TR increases for each win and decreases for each loss. When playing this mode for the first time, the player must play at least 10 ranked games to display their TR. The letter rank is based on the top percentage on the leaderboard. In order for the player to be shown on the leaderboards as well as their rank, their RD must be below 100. RD decreases every time the player plays a Tetra League game, and slowly increases over time.
The game also includes solo modes, two of which have leaderboards: 40 Lines and Blitz. For all three modes, Stride Mode can be activated. This option speeds up the countdown by changing it from "3-2-1-Go" to "Ready-Set-Go", enables tap-to-retry (by default, players will have to hold the retry key to do so), reduces the time to exit a solo game by holding the forfeit key, and speeds up all animations at the start and end of games. 40 Lines and Blitz have an option called Pro Mode which shows the finesse counter. In 40 Lines, it also displays the number of lines remaining on the board and the key presses per minute on the left.
In 40 Lines, the objective is to clear 40 lines as fast as possible.
Blitz plays identically to Ultra, with the exception of a level system present similar to Marathon with a variable line goal before increasing the level.
The player can create their own solo mode with most of the settings carried over from the Custom Play room creation menu.
Tetr.io entered the Infdev stage on January 31, 2020, when access to the game was opened for testers on a per request basis. During the Infdev release, players required to represent their IP address to osk via Discord DMs in order to test this version. A month and a half later, the game entered the public Alpha stage on March 22, 2020. This time, all players are free to create/log in to their accounts and play the game. 
On May 22, 2020, a desktop client of Tetr.io was released. Aside from being able to open the game without a web browser, this also allows users to play single-player modes offline.