The Japan Sumo Association says Yokozuna Grand Champion Hakuho has tested positive for the coronavirus.
The association says Hakuho developed abnormalities with his sense of smell and took a PCR test on Monday.
Hakuho's infection was confirmed on Tuesday, just five days before the start of the New Year Grand Sumo Tournament on Sunday.
All wrestlers who are thought to have been in contact with the yokozuna at their shared sumo stable will undergo testing. Sumo officials say they will follow the guidance of public health authorities.
Mongolia-born Hakuho won a record-extending 44th tournament title in March last year. But he missed three subsequent tournaments in the remainder of the year due to knee surgery and other factors.
In November, the Yokozuna Deliberation Council issued a rare warning to Hakuho over his repeated absences. It's the second-most severe notification following a recommendation to retire.