A bill of attainder (also known as an act of attainder or writ of attainder or bill of penalties) is an act of a legislature declaring a person, or a group of people, guilty of some crime, and punishing them, often without a trial. As with attainder resulting from the normal judicial process, the effect of such a bill is to nullify the targeted person's civil rights, most notably the right to own property (and thus pass it on to heirs), the right to a title of nobility, and, in at least the original usage, the right to life itself.

In the history of England, the word "attainder" refers to people who were declared "attainted", meaning that their civil rights were nullified: they could no longer own property or pass property to their family by will or testament. Attainted people would normally be punished by judicial execution, with the property left behind escheated to the Crown or lord rather than being inherited by family. The first use of a bill of attainder was in 1321 against Hugh le Despenser, 1st Earl of Winchester and his son Hugh Despenser the Younger, Earl of Gloucester, who were both attainted for supporting King Edward II. Bills of attainder passed in Parliament by Henry VIII on 29 January 1542 resulted in the executions of a number of notable historical figures.

The use of these bills by Parliament eventually fell into disfavour due to the potential for abuse and the violation of several legal principles, most importantly the right to due process, the precept that a law should address a particular form of behaviour rather than a specific individual or group, and the separation of powers, since a bill of attainder is necessarily a judicial matter. The last use of attainder was in 1798 against Lord Edward FitzGerald for leading the Irish Rebellion of 1798. The House of Lords later passed the Pains and Penalties Bill 1820, which attempted to attaint Queen Caroline, but it was not considered by the House of Commons. No bills of attainder have been passed since 1820 in the UK. Attainder remained a legal consequence of convictions in courts of law, but this ceased to be a part of punishment in 1870.

American dissatisfaction with British attainder laws resulted in their being prohibited in the United States Constitution in 1789. Bills of attainder are forbidden to both the federal government and the states, reflecting the importance that the Framers attached to this issue. Every state constitution also expressly forbids bills of attainder. The U.S. Supreme Court has invalidated laws under the Attainder Clause on five occasions.