Miitomo Has Been Closed Down

Hope You Saved Your Miis!

Nintendo has officially shut down the servers for Miitomo, their first mobile app. Miitomo was a communication app that used Miis to allow users to talk with friends and learn new things about them.

We want to thank you for enjoying Miitomo so much.
Miitomo’s service has come to an end as of Wednesday 9th May 2018, 16:00 (JST).
We’ve come a long way since the app was launched, and once again we want to thank everyone who’s been involved.
We hope to go on to even bigger and better things in the future with Nintendo, and we hope you’re there to share the fun.
We hope to see you again soon!

Interest in Miitomo slowly fell over time until its closure.

Source