Watanabe was a member of the USA youth and junior national volleyball teams before he decided to pursue collegiate basketball.

As a junior, he led the team to state championship, averaging 14.7 points, 8.5 rebounds, 3.2 assists and 2.9 blocks per game.

As a senior, he led the Raiders to its fourth consecutive state tournament appearance, logging a double-double average with 15.5 points and 10.2 rebounds, to go along with 4.0 assists and 3.1 blocks per game.

After redshirting his freshman year,[2][3][6] he made his collegiate debut on November 6, 2018, posting two points, six rebounds and a steal.

[3][18][12] After earning a bachelor's degree in finance, Watanabe transferred to the University of California, Davis, where he enrolled in the MBA program.

[27] On August 5, Watanabe went under the knife for the third time in less than a year, receiving a radial head replacement surgery for his injured elbow in Los Angeles.

[28] On February 5, 2023, Watanabe made his professional debut against the Toyoma Grouses, scoring seven points and grabbing six rebounds in 12 minutes of play.

[32] After Watanabe's mother sent an email to the Japan Basketball Association, he was called up to join the training camps of the national team.

[1][2][33] Watanabe made his international debut for Japan at the 2019 William Jones Cup, where he helped the team win the bronze medal.

His paternal grandfather, Doug Hogland, was an offensive linemen in the NFL for the San Francisco 49ers, Chicago Cardinals and Detroit Lions from 1953 to 1958.