Kisenosato, 30, was promoted to the top-most yokozuna rank after his win in the first tournament of the year.
He is the first Japanese-born wrestler to make it since Wakanohana in 1998. Five wrestlers from American Samoa and Mongolia have made it in the interim.
Foreign wrestlers have come to dominate sumo, amid a lack of local recruits.
Kisenosato, who comes from Ibaraki to the north of Tokyo and weighs 178kg (392 pounds), has been an ozeki - the second-highest rank - since 2012.
After being runner-up on multiple occasions, he finally clinched his first tournament victory - and thereby his promotion to yokozuna - in the first competition of 2017.
"I accept with all humility," Kisenosato said in a press conference after the Japan Sumo Association formally approved him.
"I will devote myself to the role and try not to disgrace the title of yokozuna."
Japan gets first sumo champion in 19 years
26 January 2017
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Japan has formally named its first home-grown sumo grand champion in almost two decades, in a boost to the traditional wrestling sport.
