The J-League announced a record-equaling 20 million yen ($146,000) fine for Urawa Reds on Tuesday after supporters of the first-division club were found to have repeatedly breached safety guidelines aimed at curbing the spread of the coronavirus.
The league said the fine matched the highest figure it had levied against a club and called on Urawa to ensure there were no further violations.
A group of fans cheered the arrival of the Reds' team bus at Saitama Stadium ahead of their home match against Kashima Antlers in May, ignoring the league's prohibition on vocal support, including singing and chanting.
Similar breaches occurred during a game away to Gamba Osaka this month.
Some Reds fans also did not follow the requirement to wear a mask.