OSAKA (Kyodo) -- Former WBO minimumweight boxing champion Ryuya Yamanaka, who returned to the ring after suffering a severe brain injury, has retired, his gym revealed Sunday.

The 29-year-old, who debuted as a pro in 2012 and won the world title in August 2017, retired at the age of 23 after suffering an acute subdural hematoma following his unsuccessful second title defense in July 2018.

Yamanaka returned in 2022 following the Japan Boxing Commission's revision to its regulations regarding fighters with intracranial hemorrhages.

In his last fight in June 2024, Yamanaka knocked out Thailand's Puriwat Taosuwan.

He finishes his career with a 20-4 record with seven wins by knockout.