An act of Parliament in the United Kingdom is primary legislation passed by the UK Parliament in Westminster, London.

An act of Parliament can be enforced in all four of the UK constituent countries (England, Scotland, Wales and Northern Ireland). As a result of devolution the majority of acts that are passed by Parliament increasingly only apply either to England and Wales only, or England only. Generally acts only relating to constitutional and reserved matters now apply to the whole of the United Kingdom.

A draft piece of legislation is called a bill. When this is passed by Parliament and given royal assent, it becomes an act and part of statute law.

A bill and an Act of Parliament typically include a short title and a long title, a number of clauses and, in many cases, one or more schedules. The Erskine May guide to Parliamentary Practice states that a schedule could deal with "extended material inclusion of which within clauses might detract from the sequential effect of the clauses", and would usually be incorporated through a clause in the body of the bill or act stating that each specific clause "has effect".

Acts of Parliament are classified as either "public general acts" or "local and personal acts" (also known as "private acts"). Bills are also classified as "public", "private", or "hybrid".

Public general acts form the largest category of legislation, in principle affecting the public general law applying to everyone across the entire United Kingdom, or at least to one or more of its constituent countries of England, Northern Ireland, Scotland, or Wales. Most public general acts proceed through Parliament as a public bill. Occasionally, a bill is treated as hybrid.

Private acts are either local or personal in their effect, applying to a specifically named locality or legal person in a manner different from all others. Private bills are "usually promoted by organisations, like local authorities or private companies, to give themselves powers beyond, or in conflict with, the general law. Private bills only change the law as it applies to specific individuals or organisations, rather than the general public. Groups or individuals potentially affected by these changes can petition Parliament against the proposed bill and present their objections to committees of MPs and Lords."

They include acts to confer powers on certain local authorities, a recent example being the Canterbury City Council Bill, which makes provisions relating to street trading and consumer protection in the city. Private bills can also affect certain companies: the Northern Bank Bill allowed the statutory right of Northern Bank to issue bank notes to be transferred to Danske Bank which had acquired it. Other private bills may affect particular companies established by Act of Parliament such as TSB Bank and Transas.