A ronin (浪人, rōnin?) was a samurai with no lord or master during the feudal period (1185–1868) of Japan. A samurai became masterless from the ruin or fall of his master (as in the case of death in a war), or after the loss of his master's favor or privilege.

According to the Bushido Shoshinshu (the Code of the Samurai), a samurai was supposed to commit oibara seppuku (also "hara kiri" – ritual suicide) upon the loss of his master. One who chose to not honor the code was "on his own" and was meant to suffer great shame. The undesirability of ronin status was mainly a discrimination imposed by other samurai and by the daimyo (the feudal lords).

Like regular samurai, ronin wore their two swords.

During the Edo period, with the shogunate's rigid class system and laws, the number of ronin greatly increased. Confiscation of fiefs during the rule of the third Tokugawa shogun Iemitsu resulted in an especially large increase of ronin. During previous ages, samurai were easily able to move between masters and even between occupations. They would also marry between classes. However, during the Edo period, samurai were restricted, and were above all forbidden to become employed by another master without their previous master's permission. Also, low-level samurai, often poor and without choice, were forced to quit or escape their master.