FSI is a three-letter abbreviation with multiple meanings, including:
- Foreign Service Institute, U.S. Department of State, the primary training facility for American Foreign Service Officers
- Foreign Service Institute India in New Delhi, India, the Institute for training of Indian Foreign Service Officers
- Failed States Index, a country ranking method
- Federazione Scacchistica Italiana, the Italian Chess Federation
- Financial Services Industry
- Financial Services Institution
- Financial Software Innovations
- FireSide Innocence, an alternative rock band from Lincoln, Nebraska
- FlightSafety International, an aviation training provider
- Floor Space Index, the ratio of total floor space to plot size
- Flame Spread Index, a measure for how fast flames spread in a material.
- Fluid-structure interaction, a phenomenon that occurs when a fluid interacts with a solid structure
- FontShop International, a digital type foundry
- Freeman Spogli Institute for International Studies, Stanford University's primary forum for interdisciplinary research on key international issues and challenges.
- Free Standing Insert, an advertising brochure, card, or leaflet inserted in a newspaper or magazine, usually to serve as a reply coupon or discount voucher
- Frequency Scanning Interferometry, an interferometric method of distance measurement
- FRISK Software International, an Icelandic antivirus software developer
- FSI Technologies
- Fuel Stratified Injection, a type of gasoline direct injection petrol engine made by Volkswagen Group
- Food, seed, and industrial products
- Foreign State Investment