Judo Boy (紅三四郎, Kurenai Sanshirō, "Scarlet Sanshiro") is a Japanese anime television series created by Tatsuo Yoshida and directed by his brother, Ippei Kuri (birthnamed Toyoharu Yoshida). The series aired on Fuji TV from April 2, 1969, to September 24, 1969, totaling 26 episodes. While originally based on a manga by Kuri and Yutaka Arai, published by Shueisha in 1961, the series was also followed by two manga adaptations, the first written by Yoshida and illustrted by Mamoru Uchiyama published in Shogakukan's Weekly Shōnen Sunday from July 28 to November 17, 1968, and a four-chapters series written and illustrated by Yoshida in Shueisha's Weekly Shōnen Jump from May 22 to July 10, 1969.

The series stars a teenage martial artist named Sanshiro (voiced by Ikuo Nishikawa), trained in the Kurenai School of Jiujitsu and centers around his search for his father's killer. Accompanying Sanshiro is an orphaned boy named Kenbo (voiced by Kenbo Kaminarimon) and his pet dog Boke (voiced by Hiroshi Otake). Sanshiro's only clue to his father's murderer is a glass eye left on the scene of the crime, suggesting that his father's murderer was one-eyed. Thus many of the villains Sanshiro fought during the course of the series were one-eyed or had one eye concealed with an eye-patch.

The second opening theme was performed by Mitsuko Horie (her first) when she was just 12 years old.