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).