The usually dominant Japanese had not won a medal in the class in six Olympics and world championships since 1996.
But Uchishiba displayed his speed of thought and movement in countering Jozef Krnac of Slovakia for ippon in the final to win Japan's third judo gold in two days.
China's Xian Dongmei became the first person to defeat a Japanese fighter at the Athens Games when she took the women's under 52-kg title by pinning Yuki Yokosawa just over a minute into the final.
Yokosawa made the final only by the skin of her teeth, scoring ippon in the last second of her semi-final against world champion Amarilys Savon of Cuba.
Savon took bronze by defeating Salima Souakri of Algeria with tomoe-nage (stomach throw) before bursting into tears and announcing her retirement from the sport.
Belgium's Ilse Heylen outlasted France's Annabelle Euranie in a six-minute marathon for the other bronze medal.
Cuba's Yordanis Arencibia added a men's bronze to his three world championship bronze medals after beating Georgia's David Margoshvili in 25 seconds but looked distraught at the end.
Bulgaria's Georgi Georgiev managed a small score in the golden-score period to win the battle of the underdogs in the other 66-kg bronze-medal bout against Spain's Oscar Penas.
