Yu or Yoo, or sometimes Ryu or Ryoo, is the English transcription of several Korean surnames written as 유 or 류 in hangul. As of 2000, roughly a million people are surnamed Yoo in South Korea, making up approximately 2% of the population. Of those, the most common is Ryu (Hanja: 柳, Hangul: 류),^[1] with more than six hundred thousand holders, whereas Yoo (Hanja: 兪, 余 Hangul: 유) accounts for about one hundred thousand.

The family name Yoo can be represented by any of the four hanja: 柳 (류), 劉, 兪 and 庾, each with a different meaning. In Korean, the characters 劉 and 柳 refer to 유 (Yoo) or 류 (Ryu) and are spelled as such because of the first initial sound rule (두음 법칙) in Korean, whereas the characters 兪 and 庾 refer only to 유 (Yoo). Some of these characters are used to write the Chinese surnames Liu (劉 or 柳) and Yu(兪，余）.

Notable 柳 (Ryu) clans include the Munhwa Ryu clan and the Pungsan Ryu.

This section does not cite any sources. (August 2023)

In Korea, the Yoo lineage traces to the Xia, Han, and Joseon dynasties. Holders of the surname Yoo had a reputation for charity and diligence.

The largest Ryu (which is a separate clan from Yoo, but pronounced differently), the Munhwa Ryu,^[2] was founded by a man named Ch’a Tal. Ch’a's fifth great-grandfather had been involved in an attempt to overthrow the Silla king. To avoid prosecution, the ancestor fled to Munhwa and changed his surname, first to that of his maternal grandmother, Yang, and then to Ryoo. Many years later, Ch’a Tal assisted Wang Kŏn to establish the Koryŏ Kingdom. Ch’a was recognized for his support and was rewarded accordingly. However, the historic claims that the Ch'a clans are derived from the same progenitor as Ryu Ch'a-dal have been disproven. The Munhwa Ryoo clan, along with the Andong Kwŏn clan, possess one of Korea's oldest clan genealogies. Only the character 柳 is commonly pronounced Ryu or Ryoo (류). (See Munhwa Ryoo) The surname "Ryu" comes from the character meaning willow tree. Hence, the lineage is also sometimes called "willow Ryu" (버들 류씨).

• Ryu Cha-dal (b. circa 880)
• Yu Seong-won (d. 1456), one of the Six martyred ministers during the Joseon period
• Yu Eung-bu (d. 1456), of the Gigye Yoo clan originally from Pohang, one of the Six martyred ministers during the Joseon period
• Ryu Seong-ryong (1542–1607), known for his JingBiRok
• Yu Hyeong-won (1622–1673). He was an early silhak scholar
• Yu Deuk-gong (1749–1807), historian about Balhae
• Yu Kil-chun (1856–1914), of the Gigye Yoo clan originally from Pohang; independence activist
• Yu Geun-Hyeong (1894–1993), master Korean ceramist who played a leading role in the revival of Goryeo celadon
• Il-han New (1895–1971), independence activist and entrepreneur
• Ryu Gwan-sun (1902–1920), Korean independence activist

• Liu (surname)
• Yu (Chinese name)