The Misuse of Drugs Act 1971 is an Act of Parliament, by which the United Kingdom aims to control the possession and supply of numerous drugs and drug-like substances, as listed under the Act, and to enable international co-operation against illegal drug trafficking. As passed in 1971 the Act updated UK legislation to bring it in to line with the requirements of the Single Convention on Narcotic Drugs.

The Act is presented often as little more than a list of prescribed drugs and of penalties linked to their possession and supply. In practice however the Act establishes the Home Secretary as a key player in a drug licensing system. Therefore, for example, various opiates are available legally as prescription-only Controlled Drug medicines and cannabis (hemp) may be grown under licence for 'industrial purposes'.

The Act creates three classes of "controlled substances", and ranges of penalties for illegal or unlicensed "possession" and "possession with intent to supply" are graded differently within each class. The lists of substances within each class can be amended "by order", so the Home Secretary can list new drugs and upgrade, downgrade or delist previously-controlled drugs with less of the bureaucracy and delay associated with passing an Act through both Houses of Parliament.

The Act does not cover all drugs or drug-like substances. Although, for example, cannabis is listed under the Act (as a class C drug), tobacco, another herb or plant source of drug material, is not listed.