Former sumo grand champion Harumafuji has been fined 500,000 yen ($4,400; £3,280) after being found guilty of assault.
The 33-year-old from Mongolia had admitted hitting a junior wrestler over the head with a karaoke machine remote control during a night out in Tottori in October, according to a BBC report.
He has already apologised and stepped down over the incident.
The case rocked the world of sumo, a hugely popular ceremonial sport.
The assault on fellow Mongolian Takanoiwa happened while they were out drinking with other wrestlers in a bar in the western city.
The grand champion is reported to have been angered that his countryman was checking his phone while being given advice, seeing it as showing a lack of respect.
The latter was admitted to hospital with concussion and a fractured skull.
Two others involved in the incident have faced disciplinary action, and Takanoiwa's stablemaster - as coaches are known - is expected to be demoted for allegedly delaying reporting the incident.
Harumafuji started his career in Japan at the age of 16 and was promoted to grand champion or yokozuna - sumo's highest rank - in 2012.
He released a statement in late December, Reuters reports, saying his life "is now set to be sharply different from what I thought it would be".
"I have a feeling of chagrin, to be honest. But the responsibility is all mine."
