Cài (Chinese: 蔡) is a Chinese surname that derives from the name of the ancient Cai state. In 2019 it was the 38th most common surname in China, but the 9th most common in Taiwan (as of 2018), where it is usually romanized as Tsai, Tsay, or Chai based on Wade-Giles romanization of Standard Mandarin and the 8th most common in Singapore, where it is usually romanized as Chua, which is based on its Teochew and Hokkien pronunciation. Koreans use Chinese-derived family names and in Korean, Cai is 채 in Hangul, Chae in Revised Romanization, It is also a common name in Hong Kong where it is romanized as Choy, Choi or Tsoi. In Macao and Malaysia, it is spelled as Choi, in Malaysia and the Philippines as Chua or Chuah, in Thailand as Chuo (ฉั่ว). Moreover, it is also romanized in Cambodia as either Chhay or Chhor among people of full Chinese descent living in Cambodia and as Tjoa or Chua in Indonesia.
The Chois are said to be the descendants of the 5th son of King Wen of Zhou, Ji Du. Ji Du was awarded the title of marquis (hóu) of the State of Cai (centered on what is now Shangcai, Zhumadian, Henan, China), and he was known as Cai Shu Du (“Uncle Du of Cai”). Together with Guan Shu and Huo Shu, they were known as the Three Guards. When King Wu died, his son King Cheng was too young and his uncle, the Duke of Zhou, became regent. Seeing that the power of the Duke of Zhou was increasing, the Three Guards got jealous and rebelled against Zhou together with Wu Geng. The Duke of Zhou suppressed the rebellion, and Cai Shu was exiled. King Cheng reestablished Cai Shu’s son Wu or Hu as the new Duke of Cai. Some 600 years later in the Warring States period, the State of Chu conquered Cai in 447 BC and was itself conquered by the Qin state which, in turn, formed the Qin Empire, China’s first empire. With the spread of family names to all social classes in the new empire, many people of the former state of Cai began to bear it as a surname.
The Cai descendants have undertaken the following two major migrations. During the Huang Chao Rebellion (AD 875) at the end of the Tang Dynasty (AD 618-907), the Cai clan migrated to Guangdong and Fujian provinces. Another later migration occurred when Ming Dynasty loyalist Koxinga moved military officials surnamed Cai and their families to Taiwan in the 17th century. As a result, the surname is far more common in these areas and in areas settled by their descendants (e.g., Southeast Asia) than in other parts of China.