Isao Watanabe (born 23 November 1941) is a Japanese former professional tennis player. Born in Tokyo, Watanabe competed on the international circuit in the 1960s, featuring in the main draws of the French Open and Wimbledon. He was Manuel Santana's first round opponent when the Spaniard won the 1966 Wimbledon Championships and took the opening set off him, before having to retire hurt while trailing in the second. Watanabe was the men's singles champion at the 1967 University Games in Tokyo, defeating Jun Kamiwazumi for the gold medal. He also won a silver medal in the men's doubles event.