KSI may refer to:
- Knight Companion of the Order of the Star of India
- Key Success Indicator, another name for a key performance indicator
- Killed or Seriously Injured, used in the field of road safety
- ksi (unit), kilopound per square inch, a unit of stress or pressure
- Kontoret för Särskild Inhämtning, Office for Special Assignments, Swedish intelligence agency
- the IATA code for Kissidougou airport
- The Icelandic of Football Association of Iceland, KSÍ (Icelandic: Knattspyrnusamband Íslands)
- Kerr Scientific Instruments - A manufacturer of electrophysiology equipment
- Ksi (Cyrillic letter)
- pronunciation of Xi (letter)