From Sonic Retro
April 21 (
- 1995 — Chaotix is released for the Sega 32X in Japan.
- 1999 — Issue #72 of the Sonic the Hedgehog comic series by Archie Comics is released in North America. Issue #154 of Sonic the Comic is released in the United Kingdom.
- 2003 — Sonic Darts is released for i-mode 503i compatible phones through Sega's Sonic Cafe portal in Japan.
- 2010 — Sonic the Hedgehog 2 is released for iOS in North America and Europe.
- 2020 — "Kafe Kaotikusu", the 71st episode of Sonic X, airs in Japan.
- 2022 — Sonic the Hedgehog Spinball is released for the Nintendo Switch Online service for the Nintendo Switch in North America.