At age eight Tani followed her older brother to the dojo (school for martial arts) and within months was throwing larger boys in competition. She achieved her first major victory in 1988 at the Fukuoka international women's judo tournament when she defeated the renowned Karen Briggs of England. In 1990 she captured the first of 13 consecutive titles at the Fukuoka international. Three years later she won her first world championship and received the fourth dan, the highest rank an active judo player can obtain. Tani won the silver medal in the extra-lightweight event at the 1992 Olympic Games in Barcelona, Spain, and went the next four years and 84 matches without a loss. She won her second Olympic silver medal at the 1996 Games in Atlanta, Georgia. Her first Olympic gold medal came four years later at the 2000 Games in Sydney, Australia. At the 2004 Olympic Games in Athens, Tani used an inside leg technique to defeat Frédérique Jossinet of France and to win a second gold medal.

Tani, who was widely known in Japan as “Yawara-chan” after a manga (comic book) character with whom she shares a close resemblance, enjoyed celebrity status in her homeland and was honoured by Japanese Prime Minister Junichiro Koizumi in 2002 and by Emperor Akihito in 2003. Her wedding to baseball player Tani Yoshitomo in 2003 was televised nationally.