FSI may refer to:

• Financial services industry
• Financial Secrecy Index, a ranking of secrecy jurisdictions
• Financial Stability Institute, a body of the Bank for International Settlements

• Federation of Italian Socialists (Italian: Federazione dei Socialisti Italiani), a defunct political party of Italy
• Foreign Service Institute, of the United States government
• Foreign Service Institute, India, an agency of the Indian government
• Forest Survey of India
• Fragile States Index, a country ranking method
• Internal Security Forces (French: Forces de Sécurité Intérieure), the national security force of Lebanon

• First strong isolate, a Unicode character used to support Bidirectional text
• Flame spread index, a material's propensity to burn
• Fluid-structure interaction, in fluid dynamics
• Frequency scanning interferometry, a technique for measuring the distance between two points
• FRISK Software International, an Icelandic antivirus software developer
• FRU Support Interface, a simple bus for low-level access to IBM Power-based hardware
• FSI International, an American equipment manufacturer
• Fuel stratified injection
• Full spectral imaging, in spectroscopy

• Fédération Ivoirienne du Scoutisme, the Ivorian Scouting Federation
• Ferrovie dello Stato Italiane, the Italian state-owned railway company
• FlightSafety International, an American aviation training provider
• Floor space index
• Florida Space Institute, of the State University System of Florida
• FontShop International, a German type foundry
• Fox Sports International
• Freeman Spogli Institute for International Studies, at Stanford University
• Free Sons of Israel, a fraternal organization
• IATA code for Henry Post Army Airfield, in Fort Sill, Oklahoma, United States
• Italian Chess Federation (Italian: Federazione Scacchistica Italiana)
• Free-standing insert, a type of advertising: See Sales promotion § Consumer sales promotion types