any of the masterless samurai warrior aristocrats of the late Muromachi (1138–1573) and Tokugawa (1603–1867) periods who were often vagrant and disruptive and sometimes actively rebellious.

By the 12th century the term rōnin began to be used for samurai who, as a result of either losses in battle, the untimely death of their lord, or their own misdeeds, had been dispossessed of their fief and their noble sponsorship. During the tumultuous period before the founding of the Tokugawa shogunate, their numbers increased rapidly; they remained a great cause of disorder throughout the first half of the 17th century.

In the mid-19th century many impoverished samurai were attracted to the movement to expel Western foreigners from the country and restore the old imperial family to their rightful place as the actual rulers of Japan. Large numbers of these samurai left their lords and became rōnin. These rōnin heightened the revolutionary mood of the country in the years prior to the Meiji Restoration of 1868 by assassinating moderate officials, pro-Western scholars, and foreigners residing in Japan. Although the violence continued for a short while after the restoration, the rōnin ceased to exist after samurai privileges were abolished in 1873. The affair of the 47 rōnin in the early 18th century, in which the rōnin avenged the death of their lord, has been the subject of many popular Japanese theatrical, cinema, and literary works.