Early career

Shōnannoumi made his professional debut in March 2014, alongside future ōzeki Shōdai. He was given the shikona, or ring name, "Shōnannoumi" meaning "The Sea of Shōnan", which is the region in Kanagawa prefecture where Ōiso is located. His shikona first name, Momotarō, was given to him to encourage him to defeat strong opponents, just like the character from the myth.^[2] He decided to join Takadagawa stable despite being invited by seven other stables because his master told him that the "disappointing Japanese wrestlers" would allow him to become a "hero".^[7] Shōnannoumi first served as tsukebito (assistant) to his fellow stablemate Ryūden.^[5] He would steadily rise through the lower 3 divisions and he was promoted to the makushita division in November 2016. Because of his physique, his stable had high hopes for him although Shōnannoumi suffered from the wall of the upper makushita ranks for a long time. In the July 2020 tournament, he was put up against former maegashira Ura, who was making his comeback to the top division after an injury caused him to miss 4 consecutive tournaments. Shōnannoumi ended Ura's 18-bout win streak and was Ura's only loss in this tournament.^[8] On 19 January 2021, Shōnannoumi suffered a concussion in a makushita bout against Takasago wrestler Asagyokusei. Both wrestlers bumped into each other at the tachi-ai, forcing the match to be stopped as Shōnannoumi was unable to get up and had symptoms of a concussion. After deliberating whether or not to allow him to continue, the shimpan decided on a rematch, respecting the will of Shōnannoumi to continue. Shōnannoumi won the bout but the Japan Sumo Association faced an online backlash because Shōnannoumi was obviously dazed.^[9] The Sumo Association established in the same month a new rule preventing wrestlers from competing if they have a concussion or any other physical abnormality, regardless of the wrestler's intentions.^[10]

Sekitori career

In 2022, Shōnannoumi was finally promoted sekitori after nine years in professional sumo.^[11] During the party celebrating his promotion, former Japanese Prime Minister Yoshirō Mori, a patron of his stable, declared 'I knew this man would become someone. A fine sekitori has emerged from Kanagawa Prefecture'.^[12] He initially chose a dark pink shimekomi for his first tournament because it's the colour of the Saikijin Clinic in Hiroshima who offered him the mawashi,^[3] and because he considers that color is his lucky charm.^[13] In his first tournament in jūryō, Shōnannoumi finished with a strong 12–3 record. During the May 2023 tournament, Shōnannoumi started the competition at its highest rank and finished with a score of 11–4. Looking back on his first tournaments as a sekitori, he commented that the public's expectations pushed him to do better and better in order to live up to the acclaim.^[14]

After only three tournaments in jūryō, Shōnannoumi was officially promoted to the top makuuchi division in July 2023 with the rank of west maegashira 14. He became the eleventh wrestler from his prefecture to reach this rank since Kōtokuzan in 2022, who, although born in the Philippines, was listed as coming from Yokohama (Kanagawa Prefecture), where his father originated from.^[1] He told reporters upon his promotion that he wanted to win ten matches and win a special prize at the upcoming tournament, adding that he wanted to make sure that in the weeks leading up to the July tournament he would be ready with confidence on Day 1.^[15]

On Day 12, he scored an upset victory over one of the tournament leaders, maegashira Nishikigi, and thereby achieved a kachi-koshi record.^[16] Having scored a ninth victory, Shōnannoumi was in line for one of the sanshō prize but that award was conditional on a tenth victory. On the fifteenth day of the tournament, he defeated Myōgiryū and won the award.^[17]