The House of Lords Act 1999 (c. 34) is an act of the Parliament of the United Kingdom which reformed the House of Lords, one of the chambers of Parliament. The Act was given royal assent on 11 November 1999. For centuries, the House of Lords had included several hundred members who inherited their seats (hereditary peers); the Act removed such a right. However, as part of a compromise, the Act allowed ninety-two hereditary peers to remain in the House. Another ten were created life peers to enable them to remain in the House.

The Act decreased the membership of the House from 1,330 in October 1999 to 669 in March 2000. As another result of the Act, the majority of the Lords were thence life peers, whose numbers had been gradually increasing since the Life Peerages Act 1958. As of June 2023, there were 825 members of the House of Lords, of whom 24 were senior Church of England bishops, whose representation in the House is governed by the Lords Spiritual (Women) Act 2015.

Prior to the 16th century, the Lords was the more powerful of the two houses of Parliament. A series of developments, including such moments of crisis as the English Civil War, gradually shifted the political control of England, first from the Crown to the House of Lords and then to the House of Commons. The rising wealth of the Commons eventually allowed it to wage two civil wars, dethrone two kings, and gradually reduce the power of the Lords. Prior to the House of Lords Act 1999, the power of the Lords had been diminished by the Parliament Acts 1911 and 1949 which stripped the Lords of the ability to veto most bills; at most it could delay bills for one year. Furthermore, the Commons have absolute power over money bills.

After eighteen years of Conservative government, the Labour Party led by Tony Blair won a landslide victory at the 1997 general election, inflicting the biggest defeat for the Conservatives since 1832. The Labour Party had for years endorsed abolition of the House of Lords in its election platforms, though since 1992 this had changed to a policy of reforming the House instead of such a drastic constitutional change.

During the 20th century, Liberal and Labour governments proposed many bills that were opposed by the House of Lords, which had been dominated by Conservatives since the 1890s, leading to some delay and, where proposed before elections, their dropping from the legislative agenda. In the first year of the Blair government, the Lords passed back Government bills 38 times. The rejection considered the most contentious was that of the European Elections Bill, against which the Lords voted five times. Blair stated that the Conservatives were using the hereditary peers to "frustrate" and "overturn the will of the democratically elected House of Commons". Here Blair found an opportunity to implement one of Labour's campaign promises, reforming the Lords.

On 24 November 1998, in opening the second session of Parliament, the Queen delivered her annual Speech from the Throne; the Speech was written for her by the government and outlines its legislative agenda for the forthcoming year. In it, she stated that her government would pursue a reform of the House of Lords. These remarks were followed by loud mumbles of "Hear! Hear!" from supportive Labour Members of Parliament, which led to similar utterances of "Shame! Shame!" from Conservative peers; such outbursts were unprecedented, for the Queen's Speech is usually heard by a silent Parliament.

The House of Lords Bill was expected to face a tough fight in the House of Lords. Several Lords threatened to disrupt the Government's other bills if they continued with the plan to abolish the hereditaries' right to sit in the House of Lords. Michael Onslow, 7th Earl of Onslow, for instance, said, "I'm happy to force a division on each and every clause of the Scotland Bill. Each division takes 20 minutes and there are more than 270 clauses."

Stuart Randall, Baron Randall of St Budeaux put forward the idea of phasing out the hereditary peers by disqualifying their heirs.