The black market or underground market is the part of economic activity involving illegal dealings, typically the buying and selling of merchandise or services illegally. The goods themselves may be illegal to sell (e.g. weapons or illegal drugs); the merchandise may be stolen; or the merchandise may be otherwise legal goods sold illicitly to avoid tax payments or licensing requirements, such as cigarettes or unregistered firearms.

It is so called because "black economy" or "black market" affairs are conducted outside the law, and so are necessarily conducted "in the dark", out of the sight of the law.

Black markets develop when the state places restrictions on the production or provision of goods and services. These markets prosper, then, when state restrictions are heavy, such as during a period of prohibition, price controls and/or rationing. However, black markets are currently present in any known economy.