fs or FS may stand for:

• femtosecond (fs), "femto" and "second"
• Ferrocarril de Sóller (FS), railway on Mallorca
• Ferrovie dello Stato (FS), Italian railways
• FS technology, a project studied by IBM in the early seventies.
• file system
• Francos Spain
• Fourier series
• FS, a Mazda piston engine
• Flagship Studios, a computer game company
• Fuck Society, a humor website
• "French ship", a NATO prefix for ships of the French Navy
• Fin-stabilized projectiles, such as APFSDS or HEAT-FS ammunition for a tank gun.