Japan’s ancient sport has a new grand champion in its midst. The 293-pound Harumafuji was elevated to the highest rank of sumo on Wednesday to become only the 70th wrestler to be named grand champion in the sport’s centuries-long history.

His promotion became a near foregone conclusion after he clinched his second consecutive tournament over the weekend. On Sunday, the 28-year-old closed out the Autumn Grand Sumo tournament with a perfect record. On his way to victory, he defeated Hakuho, the superstar wrestler who until this...