Si or si or SI may stand for:
- Si, the lunar deity in Mohican mythology
- si, one of Spanish language words pronounced sē, like "see":
- Si, the seventh note of solfege
- SI, the Système International d'Unités, the (metric) International System of Units
- silicon (Si, symbol for the chemical element)
- Slovenia (ISO country code)
- Smithsonian Institution
- Socialist International
- Sports Illustrated
- Staten Island, a borough of New York City and an island in the Hudson River
- Swedish Institute (Svenska institutet)