The game is a multiplayer version of Tetris that puts the player up against 98 other players online in a last-man standing form of Battle Royale. Every time the player clears a line, garbage is sent to another player's field that stacks up at the bottom and renders that area of the field unusable along with leaving that player in more danger of reaching over the top of the field and getting a Game Over. To eliminate garbage, the player must add blocks that are falling into spaces within the garbage field to clear the lines. The player can see at all times which other player is sending them garbage and can choose which player they send garbage to.
In May of 2019, Nintendo released a Paid DLC for Tetris 99 known as the Big Block DLC which added four new offline and local modes. In CPU Battle the player can battle 98 CPU players. In Marathon, the player plays an endless game of Tetris with the goal of achieving a new high score. Local Arena allows up to eight players to play together via local wireless, and Two-Player Share Battle has two players using separate Joy-Cons playing on the same field.
Since its release, the game has featured regular Maximus Cups which offer rewards to players who can win top scores over repeated online matches. The first Maximus Cups offered My Nintendo Club rewards but every cup starting with the May 2019 cup offered special in-game themes based on other Nintendo Switch games to players who earned enough in-game points by playing repeated matches.
The following tropes are connected to Tetris 99:
- Anti-Frustration Features: The game lets you see exactly which player is currently targeting you with garbage in addition to letting you choose which player you are targeting with garbage.
- Background Music Override: Every theme including the default theme changes the music when the match is down to 50 players and changes again when down to the very last ten.
- Crossover: Several of the MAXIMUS Cups for Tetris 99, beginning with the fifth, award participants with special skins based on other games (usually Nintendo ones) featuring unique music and sound effects once they accumulate enough points. These are usually dualvertisements for the release of a game or a wider promotional event. So far, there's been Splatoon 2 (with songs and sound effects from the first game), Fire Emblem: Three Houses, Super Kirby Clash, Luigi's Mansion 3, Pokémon Sword and Shield, Ring Fit Adventure, Animal Crossing: New Horizons, Xenoblade Chronicles: Definitive Edition, Paper Mario: The Origami King, Super Mario All-Stars, Kirby Fighters 2, Super Mario 3D World + Bowser's Fury, Miitopia, Mario Golf: Super Rush, The Legend of Zelda: Skyward Sword HD, WarioWare: Get It Together!, Monster Hunter: Rise, Metroid Dread, Mario Party Superstars, Pokémon Legends: Arceus, Kirby and the Forgotten Land, Kirby's Dream Buffet, Fire Emblem Engage, Kirby's Return to Dream Land Deluxe, Pikmin 4, Xenoblade Chronicles 3, Super Mario RPG Remake, WarioWare: Move It! and Super Mario Bros. Wonder themes. The 2.0.0 update also has unlockable skins based on Super Mario Bros., The Legend of Zelda, and Donkey Kong.
- Freemium: The game was offered as a free download to members of the Switch Online service but later got the Big Block paid DLC.
- Misbegotten Multiplayer Mode: While the game is built around online battles with up to 98 opponents, and it works well on that front, the local multiplayer modes leave something to be desired. 2P Share Battle allows two players to play on the same device...but the two players' playfields are arranged vertically rather than the traditional horizontal arrangement, and since Tetris playfields have a 1:2 aspect ratio, this means each player's share of the screen will be quite small, especially if playing undocked on the Switch's built-in screen. Local Arena allows up to 8 players to play via local wireless connection, and here at least each player has a full-sized playfield. Both modes still add CPU opponents to have 99 entrants (97 CPUs for Share Battle and 91-97 for Local Arena), so you can't have a local multiplayer match that's just between you and other humans.
- Mythology Gag:
- The very first special theme from a Maximus Cup was based on the very first Tetris on the Game Boy in 1989 as a celebration of that game's 30th anniversary.
- The Mario Party Superstars theme initially uses the music from the "Mario's Puzzle Party" minigame, which is a simplified puzzle game.
- Public Domain Soundtrack: As usual, "Korobeiniki" appears as the music for the default theme. A high intensity remix of "Flight of the Bumblebee" serves as the music for when 10 players remain in the same theme.
- Songs in the Key of Panic: Once 10 players remain, the music becomes much more intense.
- Soundtrack Dissonance: The Animal Crossing: New Horizons theme lifts music directly from its source game, which doesn't really loan itself well to the high intensity of this game.
