Monocoque ( MON-ə-ko(h)k), also called structural skin, is a structural system in which loads are supported by an object's external skin, in a manner similar to an egg shell. The word monocoque is a French term for "single shell".

First used for boats, a true monocoque carries both tensile and compressive forces within the skin and can be recognised by the absence of a load-carrying internal frame. Few metal aircraft other than those with milled skins can strictly be regarded as pure monocoques, as they use a metal shell or sheeting reinforced with frames riveted to the skin, but most wooden aircraft are described as monocoques, even though they also incorporate frames.

By contrast, a semi-monocoque is a hybrid combining a tensile stressed skin and a compressive structure made up of longerons and ribs or frames. Other semi-monocoques, not to be confused with true monocoques, include vehicle unibodies, which tend to be composites, and inflatable shells or balloon tanks, both of which are pressure stabilised.