Cài (Chinese: 蔡) is a Chinese-language surname that derives from the name of the ancient Cai state.

Koreans use Chinese-derived family names and in Korean, Cai is 채 in Hangul, "Chae" in Revised Romanization,[3] It is also a common name in Hong Kong where it is romanized as "Choy", "Choi" or "Tsoi".

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.

Another later migration occurred when Ming dynasty loyalist Koxinga moved military officials surnamed Cai and their families to Taiwan in the 17th century.

(This should not be confused with the predominantly Korean family name Choi which has a different character [崔]).

Cai is romanized as Cai in the People's Republic of China, Tsai (or occasionally Tsay or Chai for Mandarin) or Tsoa in the Republic of China (Taiwan), and Choi or Choy in Hong Kong and Malaysia.

Hispanicized forms of the name include Chuachiaco, Chuakay, Chuapoco, Chuaquico, Chuacuco, Tuazon, Chuateco, and Chuatoco.

Legislation by Siamese King Rama VI (r. 1910–1925) required the adoption of Thai surnames which was largely directed at easing tensions with Chinese community by encouraging assimilation.

For example, Indonesianized forms of Cai include Tjuatja, Cuaca, Tjuandi, Cuandi, Tjahjana, Tjahja, etc.

Despite the Indonesianization, the Chinese surnames are still used today by the Chinese-Indonesian diaspora overseas (mostly in the Netherlands, Germany, and USA); by those Chinese-Indonesians courageous enough during Suharto's regime to keep their Chinese names (e.g., Kwik Kian Gie), or by those who couldn't afford to process the name change through Indonesia's civil bureaucracy.