Oiran (花魁) is a collective term for the highest-ranking courtesans in Japanese history, who were considered to be above common prostitutes (known as yūjo (遊女, lit. 'woman of pleasure')) for their more refined entertainment skills and training in the traditional arts. Divided into a number of ranks within this category, the highest rank of oiran were the tayū, who were considered to be set apart from other oiran due to their intensive training in the traditional arts and the fact that they lived and worked in Kyoto, the political capital of Japan, which remained the cultural heart of the country when the seat of political power moved to Tokyo. Though oiran by definition also engaged in prostitution, higher-ranking oiran had a degree of choice in which customers they took.

The term oiran originated in Yoshiwara, the red light district of Edo in the 1750s, and is applied to all ranks of high level courtesans in historical Japan.

The services of oiran were well known for being exclusive and expensive, with oiran typically only entertaining the upper classes of society, gaining the nickname keisei (lit. 'castle toppler') for their perceived ability to steal the hearts and match the wits of upper-class men. Many oiran became celebrities both inside and outside of the pleasure quarters, and were commonly depicted in ukiyo-e woodblock prints and in kabuki theatre plays. Oiran were expected to be well versed in the traditional arts of singing, classical dance and music, including the ability to play the kokyū and the koto, and were also expected to converse with clients in upper class and formalised language.

Though regarded as trend setting and fashionable women at the historic height of their profession, this reputation was later usurped in the late 18th through 19th centuries by geisha, who became popular among the merchant classes for their simplified clothing, ability to play short, modern songs known as kouta on the shamisen, and their more fashionable expressions of contemporary womanhood and companionship for men, which mirrored the tastes of the extremely wealthy, but for lower class merchants, who constituted the majority of their patronage.

The popularity and numbers of oiran continued to decline steadily throughout the 19th century, before prostitution was outlawed in Japan in 1957. However, the tayū remaining in Kyoto's Shimabara district were allowed to continue practising the cultural and performing arts traditions of their profession, and were declared a "special variety" of geisha. In the present day, a handful of tayū, who do not engage in prostitution as part of their role, continue to perform in Kyoto, alongside a number of oiran reenactors elsewhere in Japan who perform in reenactments of the courtesan parades known as oiran dōchū.

The word oiran comes from the Japanese phrase "oira no tokoro no nēsan" (おいらの所の姉さん) which translates loosely to "the sister at our (my) place." When written in kanji, the word consists of two characters: 花, meaning "flower", and 魁 meaning "leader". Though only the highest ranking prostitutes of Yoshiwara were technically known as oiran, the term is now widely applied to all.

Compared to yūjo, whose primary attraction was the sexual services they offered, oiran, and particularly tayū, were first and foremost entertainers. In order to become an oiran, a woman first had to be educated in a range of skills from a relatively young age, including sadō (Japanese tea ceremony), ikebana (flower arranging) and calligraphy. Oiran also learned to play the koto, shakuhachi, tsuzumi (hand drum), shamisen and kokyū. Clients expected oiran to be well read, able to converse and write with wit and elegance, and able to match them in intellect in conversation.

Within the pleasure quarters, an oiran's prestige was based on her beauty, character, education and artistic ability, which was reflected in the number of ranks falling in the category of oiran. An oiran, unlike geisha or common prostitutes, could be promoted or demoted by the owner of her brothel, and commonly inherited a generational name (名跡, myōseki) upon gaining promotion to a higher rank; these names, exclusively the property of the brothel owner, typically carried the prestige of the person who held it previously, and brothel owners commonly chose only those of similar countenance and reputation to inherit them. Myōseki were written in kanji, and were typically more elaborate than the average woman's name of the time, holding meanings taken from poetry, literary history and nature; myōseki were rarely passed from one oiran directly down to their apprentice.