The W55 was an American thermonuclear warhead used as the warhead for the SUBROC antisubmarine rocket system. The W55 was designed in the early 1960s, and produced from 1964 to 1968. The last units were retired from service in 1990. A total of 285 W55 weapons were produced.
The W55 reportedly had a yield of 1 to 5 kilotons, which is very low for a two-stage thermonuclear device and may only refer to the yield of the primary stage. Some sources suggest that the W55 evolved from the experimental bomb tested in the Hardtack I Olive nuclear test on July 22, 1958, which had a full two-stage yield estimated at 202 kilotons.
The W55 was 35 centimetres (14 in) in diameter, 1 metre (39 in) long, and weighed 213 kilograms (470 lb).
Researcher Chuck Hansen claims based on his US nuclear program research that the W55 and W58 warheads shared a common primary or fission first stage, and that this design was nicknamed the Kinglet primary by Hansen in 2001.