F91

F91 may refer to : and also:
 * F91 Dudelange, a football club, based in Dudelange in southern Luxembourg
 * Mobile Suit Gundam F91, a 1991 animated film
 * HMS Brazen (F91), a 1981 British Royal Navy Type 22 frigate
 * HMS Murray (F91), a British Royal Navy Blackwood class second-rate anti-submarine frigate
 * Republic XF-91 Thunderceptor, a rocket-jet hybrid interceptor
 * Casio F-91W, a common and inexpensive digital watch, claimed by some to be associated with terrorism
 * The McCarthy 91 function, a recursively defined mathematical function
 * Conduct disorders ICD-10 code