Plastic explosive (or plastique) is a specialised form of explosive material. It is soft and hand malleable and may have the added benefit of being usable over a wider temperature range than pure explosive. Plastic explosives are especially suited for explosive demolition as they can be easily formed into the best shapes for cutting structural members, and have a high enough velocity of detonation and density for metal cutting work. They are generally not used for ordinary blasting as they tend to be significantly more expensive than other materials that perform just as well in that field. Also, when an explosive is bound in a plastique, its power is generally lower than when it is pure.

One of the earliest plastic explosives was Nobel's Explosive No. 808, known also as "Nobel 808", developed by the British company Nobel Chemicals Ltd. well before World War II. It had the appearance of green plasticine with a distinctive smell of almonds. During WW2 it was extensively used by the British Special Operations Executive (SOE) for sabotage missions. It is also the explosive used in HESH anti-tank shells.

During and just after World War II a number of new RDX based explosives were developed, including Compositions C, C2, and eventually C3. Together with RDX these incorporated various plasticisers to decrease sensitivity and make the composition plastic.

The origin of the US term plastique is due to the plastic explosive introduced to the US by the British in 1940. The samples of explosive brought to the USA by the Tizard Mission had been packaged by SOE ready for dropping to the French Resistance and were labelled in French, as Explosif Plastique.

C3 was very effective but proved to be too brittle in cold weather. In the sixties it was replaced by C-4, also using RDX but with polyisobutylene and di(2-ethylhexyl)sebacate as the binder and plasticizer.

Semtex was also developed in the 1960s by Stanislav Brebera by mixing of RDX with PETN and then adding binders and stabilizers.