UI or Ui may refer to:
In science and technology:
- An alternate symbol for the international unit (IU)
- Unit interval (data transmission)
- Universal indicator, a type of pH indicator
- Unit Injector, a fuel injection technology for diesel engines
- Urinary incontinence
- User interaction
- User interface
- Ui (digraph), a digraph used in some writing systems
- Unemployment Insurance
In education:
- University of Iceland - Reykjavík, Iceland
- University of Innsbruck - Innsbruck, Austria
- University of Indonesia - Depok, West Java and Salemba, Jakarta, Indonesia
- University of Ibadan - Ibadan, Nigeria
- University of Idaho - Moscow, Idaho
- University of Illinois - Urbana-Champaign, Chicago, and Springfield, Illinois
- University of Iowa - Iowa City, Iowa
- University of Isfahan - Isfahan, Iran
- Universities Ireland
In companies and associations:
- The United Illuminating Company
- Eurocypria Airlines, IATA airline designator
- Unix International
Other:
- Ui (band), an American post-rock band
- United Ireland
- Utility infielder, baseball player
- Ui, an ancient Phoenician colony located at the site of modern-day Tripoli in Libya
- Arturo Ui, a fictional character from The Resistible Rise of Arturo Ui by Bertolt Brecht
- Ui Hirasawa, a fictional character in the anime and manga series K-On!