Watanabe was born in 1994 in Kagawa, Japan. Both of his parents played in Japan's top basketball leagues. Before coming to the United States, he led Jinsei Gakuen High School to second place in the 2010 and 2011 All-Japan Tournaments. He moved to the USA in 2013, playing his only scholastic season in the States at St. Thomas More High School in Connecticut. In signing with George Washington University, Watanabe became the first Japanese-born student-athlete to earn an NCAA Division I men's basketball scholarship. During college, he majored in Japanese Language and Literature. Fans can check out Watanabe on Instagram (@yutawatanabe12). Watanabe was named the Atlantic 10 Defensive Player of the Year in his senior year at George Washington for the 2017-18 season. As a junior in 2016-17, he was named to the Atlantic 10 All-Defensive Team. The Kagawa, Japan native was third in the Atlantic 10 in blocks (54) and blocks per game (1.6) in his senior season. Along with those categories, Watanabe led George Washington in points per game (16.3), defensive rebounds per game (4.8) and field goals per game (5.8). Watanabe was not selected in the 2018 NBA Draft. He signed a contract with the Nets not long after in the summer of 2018.