(Japanese: “tough gang”), any of various Japanese criminal gangs of centuries-long tradition, which combined in the 20th century into Mafia-like organizations. Members, often called yakuza (“good-for-nothing”), or gyangu (“gangster”), adopt samurai-like rituals and often bear elaborate body tattoos. They engage in extortion, blackmail, smuggling, prostitution, drugs, gambling, loan sharking, day-labour contracting, and other rackets and control many restaurants, bars, pachinko parlours, trucking companies, talent agencies, taxi fleets, factories, and other businesses in major Japanese cities. The bōryokudan were also involved in criminal activities in the United States.

The bōryokudan date to the 16th century, when unemployed samurai turned to banditry, often gathering into small gangs. By the late 20th century their numbers exceeded 150,000 (according to police estimates), organized into more than 2,000 gangs—most of them affiliated under the umbrella of one of a dozen or fewer conglomerate gangs. The largest conglomerates include the Yamaguchi-gumi, founded about 1926 by Yamaguchi Harukichi but fully developed and aggrandized only after World War II by Taoka Kazuo (q.v.); Inagawa-kai; and Sumiyoshi-Rengo.

The leader of any gang or conglomerate of yakuza is known as the oyabun (“boss”) and the followers are known as koban (“proteges,” or “apprentices”); the rigid hierarchy and discipline of the bōrykudan are usually matched by a right-wing, ultranationalistic ideology. Kobun take a blood oath of allegiance, and a member who breaks the yakuza code must show penance—often ritualistically by cutting off his little finger with a sword and presenting it, wrapped in a silk scarf, to his oyabun.