F31 or F.31 may refer to:

Aircraft

• Farman F.31, a French prototype fighter aircraft

Automobiles

• BMW 3 Series (F31), a German estate car
• Nissan Leopard F31, a Japanese sedan

Ships and boats

• F-31 Sport Cruiser a trimaran sailboat
• HMS Cyclops (F31), a submarine depot ship of the Royal Navy
• HMS Mohawk (F31), a Tribal-class destroyer of the Royal Navy
• INS Brahmaputra (F31), two frigates of the Indian Navy

• F31 (classification), a disability sport classification for seated throwing events
• Bipolar affective disorder
• Fluorine-31 (³¹F), an isotope of fluorine

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