As promised, the company's first foray into smartphone apps is a free-to-play social title in which players can create and customise their own avatars, known as Miis, which can communicate with other Miis on the social network.
Apart from the social angle, the main incentive to play is to earn points which can be exchanged for discounts on console and handheld games, console themes and other rewards through the company's new loyalty program, My Nintendo, which also launched in Japan alongside the game.
MacRumors sister site Touch Arcade has posted its first impressions on Miitomo, which is currently limited to the Japanese App Store but expected to roll out to most countries by the end of the month.
If you simply can't wait, Touch Arcade has also posted a helpful guide explaining how to download games from other regions.
Nintendo has promised more games for smartphones, some of which are expected to be pay-to-download. Registrations have also gone live for Nintendo's new multiplatform account system, Nintendo Account, which connects PC, Nintendo hardware and smartphone users, and enables the transfer of game data between mobile and console games.
Update: Nintendo has announced that Miitomo launches in the U.S. and "several other countries" on March 31.