SI (disambiguation)

SI, Si, or si may refer to:

 Science 
 Silicon, chemical symbol Si
 International System of Units, abbreviated SI from its French name, Système International d'unités
 Self-injury
 Sexual intercourse, a medical abbreviation
 Suicidal ideation, a medical term for thoughts about suicide
 Small intestine, part of the digestive system
 .si, the Internet country code top-level domain for Slovenia

 Places 
 Slovenia, a European country (from the ISO 3166-2 two-letter abbreviation)
 Solomon Islands, a Pacific island country
 South India, a region in India
 The South Island, one of the two main islands of New Zealand
 Southern Ireland, an old name for the Republic of Ireland
 Staten Island, a borough of New York City and an island in the Hudson River
 Province of Siena, an Italian province (two letter provincial code)

 Other uses 
 Sí (album), an album by Julieta Venegas
 Smithsonian Institution, a research institute
 Socialist International, an organisation of socialist and labour political parties
 Societas Iesu, the Jesuits, a group within the Roman Catholic Church
 Sports Illustrated, a sports magazine