BSI is a three letter acronym that can stand for:
- Backside illumination, a type of digital image sensor
- The Baker Street Irregulars, a fictional group featured in Sherlock Holmes stories
- Banca della Svizzera Italiana
- Bentley Systems Inc.
- Bible Society of India
- Bloodstream infection
- Board of Special Investigation
- Body substance isolation
- Botanical Survey of India
- Bridge Software Institute, University of Florida
- British Solomon Islands, a former territory
- British Standards Institution aka BSI Group, erroneously, 'British Standards Institute'
- Broadcast signal intrusion
- Build Security In, a US federal initiative
- Bundesamt für Sicherheit in der Informationstechnik, the German Federal Office for Information Security
- Business Systems Infrastructure
- BSI coupling, a railroad coupling or coupler
Also, in certain French cars the term BSi may refer to the Body Control Module.