The term jig in mechanical engineering refers to a type of tool, often single-purpose, used to check assembly or disassembly, for handling products or determining correct product dimensions (e.g. so it is not necessary to re-measure the product if production is repeated).

Jigs are usually more expensive to purchase, which is why the great majority of jigs used in repeated production in particular tend to be single-purpose. A jig is also purchased even for making a small number of products, if the requisite quality cannot be attained without it. A product cannot be safely handled without a jig, for example, in which case more universal, adjustable jigs are used

Jigs are commonly used for assembly and disassembly, welding, bonding, drilling, punching, press bending, handling and storage, measurement, and other purposes. The tool and jig designer is a specialised profession in in mechanical engineering.