Riichi is the defining declaration of riichi mahjong. Under Tenhou rules, a player whose hand is closed and in tenpai may declare by turning their discard sideways and placing a 1,000-point deposit, provided they have at least 1,000 points and another draw remains possible. WRC 2025 instead allows riichi below 1,000 points and with fewer than four live-wall tiles. A ron on the declaration discard voids the riichi and its deposit.
Declaring riichi grants a yaku and opens the door to uradora. Afterward, every drawn tile must normally be discarded if it does not complete the hand. Under Tenhou rules, an ankan remains legal with the tile just drawn when the wait stays identical, even if structure or yaku change. WRC 2025 also requires the hand structure to remain unchanged.
Riichi also enforces strict defensive discipline: after declaring, the player becomes exposed to furiten and can no longer fold. Timing the riichi well, balancing value against safety, is one of the deepest decisions in the game.