The Reform Acts (or Reform Bills, before they were passed) are legislation enacted in the United Kingdom in the 19th and 20th century to enfranchise new groups of voters and to redistribute seats in the House of Commons of the Parliament of the United Kingdom. When short titles were introduced for these acts, they were usually Representation of the People Act.

These began with the Reform Act 1832, Reform Act 1867, and the Representation of the People Act 1884, to increase the electorate for the House of Commons and remove certain inequalities in representation. The bill of 1832 disfranchised many boroughs which enjoyed undue representation and increased that of the large towns, at the same time extending the franchise. It was put through Parliament by the Whigs. The bill of 1867 was passed by the Conservatives under the urging of the Liberals, while that of 1882 was introduced by the Liberals and passed in 1884. These latter two bills provided for a more democratic representation.

Following the First World War, the Reform Act 1918 was enacted with cross-party unanimity. It enfranchised all men over the age of 21 and women over the age of 30. Ten years later, the Reform Act 1928, passed by the Conservatives, resulted in universal suffrage with a voting age of 21. In 1969, the United Kingdom became the first major democratic country to lower its age of franchise to 18 in the Reform Act 1969 passed by the Labour government.

Internationally, the Parliament of the United Kingdom and its Westminster system played a "vanguard role" with worldwide influence on the spread of democracy, thus it is often known as "The mother of parliaments".

In the United Kingdom of Great Britain and Ireland, before 1832, fewer than one adult male in ten was eligible to vote in parliamentary elections. Moreover, the franchise varied a great deal between England (which included Wales), where it was wider, and Scotland and Ireland, where it was narrower. A few boroughs gave the vote to all male householders, but many parliamentary seats were under the control of a small group or sometimes a single rich aristocrat. Reforms had been proposed in the 18th century, both by radicals such as John Wilkes and by more conservative politicians such as William Pitt the Younger. However, there was strong opposition to reform, especially after the outbreak of the French Revolution (1789–1799). The cause was continued after 1792 by the London Corresponding Society.

Eventually, the parliamentary franchise was expanded and made more uniform through a series of Reform Acts beginning with the Great Reform Act in 1832. These acts extended voting rights to previously disenfranchised citizens. Sources refer to up to six "Reform Acts", although the earlier three in 1832, 1867/68 and 1884, are better known by this name. Some other acts related to electoral matters also became known as Reform Acts.

The following Acts of Parliament are known as Reform Acts:

There are many other electoral reform Acts that changed the electoral system in the United Kingdom. Such legislation typically used "Representation of the People Act" as the short title, by which name the 1918, 1928 and other acts in the 20th century are better known. The title Representation of the People Act was adopted in other countries of, or formerly part of, the British Empire through the spread of the Westminster parliamentary system. The Parliament of the United Kingdom played a "vanguard role" with worldwide influence on the spread of democracy, thus it is often known as "the mother of parliaments".