USI may stand for:
- User System Interaction, a postgraduate engineering design program in the Netherlands
- U Select It, a large vending company in the United States
- Union of Independent Trade Unions (Portugal)
- Union of Students in Ireland, the representative body for students' unions in Ireland
- Unione Sindacale Italiana, an Italian trade union
- United States of Indonesia, a state in Southeast Asia from 1949 to 1950
- University of Southern Indiana, a University located in Evansville, Indiana, United States
- Università della Svizzera italiana, the University of Italian-speaking Switzerland
- Unlimited Software Inc., see Distinctive Software