Originally written in 2025-04:
Riichi mahjong is a four player gambling game similar to poker in feel, but much more complex.
Players draw tiles in turns, trying to complete their hands by collecting multiple tile groups similar to full poker hands, such as a flushs, straights, or pairs. Hands can range hugely in value by how difficult they are to build.
The greatest difference to poker is perhaps the constant need to monitor hands of your opponents. Players keep their hands hidden but each turn they must discard one tile openly, which provides other players with indirect information about what hands they might be going for and which not. If a player fails to identify which tiles could complete hands of their opponents and discard such a tile, then he will have to pay the winnings of that player all by himself. This makes the gameplay very defensive in nature, meaning it's less about trying to win and more about trying to lose the least. Various rules make dealing into hands even more punishing which increases the gambling aspect further.
As in poker there is a large luck factor to the game that evens out when many matches are played in succession. Still, it can be very frustrating not knowing if you lost because of bad luck or lacking skill.
The image shows replay tool of Tenhou. The game has resulted in a draw where every player has an outrageously valuable hand missing just one tile. A game like this is so unlikely to happen that I would not be surprised if the image is a fabrication.
The game is mostly played in Japan and as such the online servers may not offer English as an option. However, with help of a guide or two it's actually not that difficult to play on those servers even if you can't read the language.