B?s?zoku ("violent running gang") is a Japanese subculture associated with motorcycle clubs and gangs.

They were first seen in the 1950s as the Japanese automobile industry expanded rapidly. The first b?s?zoku were known as kaminari-zoku ("Lightning Tribes"). It is common to see b?s?zoku groups socializing in city centers and playing loud music characterized by their lifestyle, such as The Roosters, and the Street Sliders.

The word b?s?zoku is also applied to motorcycle gangs, who share an interest in modifications (often illegal) for motorcycles, such as removing the mufflers so that more noise is produced. These b?s?zoku groups also engage in dangerous or reckless driving, such as weaving in traffic, not wearing motorcycle helmets, and running red lights. Another activity is shinai b?s? speeding in city streets, not usually for street racing but more for thrills. With many bikes involved, the leading one is driven by the sent?sha, the leader, who is responsible for the event and is not allowed to be overtaken. Japanese police call them Maru-S? (police code ???), and dispatch a police vehicle to trail any groups of bikes to prevent any possible incidents, which can include riding through suburbs at speeds of 5–10 miles an hour, creating a loud disturbance and waving imperial Japanese flags, to starting fights which can include weapons such as wooden swords, metal pipes and Molotov cocktails. These b?s?zoku gangs are generally composed of people under the legal adult age, which in Japan is 20 years old, and are subject to increasing state and police pressure.