[9] In December 2018, he became the youngest player to win the ITTF World Tour Grand Finals at the age of 15 years and 172 days.

Aged 12 years and 355 days, Harimoto defeated seasoned professionals Ho Kwan Kit, Hugo Calderano, and teammate Kohei Sambe to win the 2016 U-21 Japan Open title.

[1] Later that year, Harimoto won gold medals in the boys' singles and teams events at the World Junior Table Tennis Championships in Cape Town, South Africa.

This win was historic, as Harimoto became the youngest winner of the World Junior Championships aged 13 years and 163 days.

He reached the finals with victories over Álvaro Robles, Sakai Asuka, Robert Gardos, and local favorite Sharath Kamal, before losing to defending champion Dimitrij Ovtcharov in straight sets.

[21] In June, teammate Jun Mizutani said that Harimoto's mental game was steadily improving in 2021 and better than the previous year.

[22] Harimoto was upset by Darko Jorgic in the round of 16 of the men's singles event at the Tokyo Olympics.

[23] Originally slated to be the ace player in the team event, Harimoto ended up playing in doubles in Japan's 3–1 victory against Sweden in the quarter-finals.