The B43 was a United States air-dropped nuclear weapon used by a wide variety of fighter bomber and bomber aircraft.

The B43 was developed from 1956 by Los Alamos National Laboratory, entering production in 1959. It entered service in April 1961. Total production was 2,000 weapons, ending in 1965.

The B43 was built in two variants, Mod 1 and Mod 2, each with five yield options. Depending on version, length was between 3.81 m (12 ft 6 in) and 4.15 m (13 ft 8 in), with a weight of 934 kg (2,060 lb) to 962 kg (2,125 lb). It could be delivered at altitudes as low as 61 m (300 ft), with fusing options for airburst, ground burst, free fall, contact, or laydown delivery. Explosive yield varied from 70 kilotons to 1 megaton.

Carrier aircraft included most USAF and USN fighters and bombers, including the A-4 Skyhawk, A-6 Intruder, A-7 Corsair II, B-47 Stratojet, B-52 Stratofortress, F-4 Phantom II, F-15 Eagle, F-16 Fighting Falcon, F-105 Thunderchief, F/A-18 Hornet, and FB-111A . (The B-1B Lancer was intended to carry the B43 as well, but it is unclear if it was ever cleared.)

The B43 was never used in anger, but it was involved in a nuclear accident when an A-4 Skyhawk of the USS Ticonderoga (VA-56) was lost off the coast of Japan on 5 December 1965. Neither the aircraft, the pilot, nor the bomb were found.

The B43 was phased out in the 1980s, and the last B43 weapons were retired in 1991 in favor of the newer B61 and B83 weapons.