Onosato, 24, promoted to sumo grand champion in record time
The 24-year-old's elevation by the Japan Sumo Association came after he appeared in just 13 professional tournaments, eight fewer than the current record holder.
The wrestler, who weighs 191kg (421lb, 30 stone) and is 1.92 metres (6ft 3in) tall, became the 75th grand champion in the centuries-old history of sumo.
He was the second to reach the top rank in the space of four months, following Hoshoryu's promotion in January.
The two will go head-to-head for the first time as yokozuna in July in Nagoya.
Onosato also became the first Japan-born wrestler to reach the rank since 2017. Six of the last seven grand champions were born in Mongolia.
Onosato was recommended for promotion on Monday after he won the Summer Grand Sumo Tournament a day earlier for his second straight title and the fourth of his career.
"I am truly happy. Now, things will become more important than ever. I want to stay focused and continue to work hard," he told a press conference after being formally promoted.
"This is a very much unknown territory for me. I want to maintain my style, be Onosato, and I will work hard to become a unique and unparalleled yokozuna."
There are no set criteria for promotion from the second-highest rank of ozeki, and cases are considered by the Yokozuna Deliberation Council.
Hoshoryu's promotion in January meant that sumo avoided having no grand champion for the first time in more than 30 years.
Onosato's promotion was the fastest since the current six-tournament calendar was introduced in 1958.