The Enforcement Act of 1871 (17 Stat. 13), also known as the Ku Klux Klan Act, Third Enforcement Act, Third Ku Klux Klan Act, Civil Rights Act of 1871, or Force Act of 1871, is an Act of the United States Congress that was intended to combat the paramilitary vigilantism of the Ku Klux Klan. The act made certain acts committed by private persons federal offenses including conspiring to deprive citizens of their rights to hold office, serve on juries, or enjoy the equal protection of law. The Act authorized the President to deploy federal troops to counter the Klan and to suspend the writ of habeas corpus to make arrests without charge.

The act was passed by the 42nd United States Congress and signed into law by President Ulysses S. Grant on April 20, 1871. The act was the last of three Enforcement Acts passed by Congress from 1870 to 1871 during the Reconstruction era to combat attacks upon the suffrage rights of African Americans. The statute has been subject to only minor changes since then, but has been the subject of voluminous interpretation by courts.

This legislation was asked for by President Grant and passed within one month of when he sent the request to Congress. Grant's request was a result of the reports he was receiving of widespread racial threats in the Deep South, particularly in South Carolina. He felt that he needed to have Congress delegate broader authority to the President before he could effectively intervene. After the act's enactment, the president had the power for the first time to both suppress state disorders on his own initiative and to suspend the writ of habeas corpus. Grant did not hesitate to use this authority on numerous occasions during his presidency, and as a result the KKK was completely dismantled (ending the "first Klan" era) and did not resurface in any meaningful way until the beginning of the 20th century.

Several of the act's provisions still exist today as codified statutes. Congress delegated to the federal judiciary the authority to enforce violations of civil rights, with the most important of these enabling statutes being section 1979 of the Revised Statutes ('42 U.S.C. § 1983) entitled as Civil action for deprivation of rights'. It is the most widely used civil rights enforcement statute, allowing people to sue in civil court over civil rights violations.

H.R. 320 was brought to the floor of the US House for a vote on April 7, 1871. Four parties were present for the vote. Republicans voted 115 in favor, 0 against. Democrats voted 0 in favor, 91 against. Liberal Republicans voted 2 in favor 0 against. Independent Republicans voted 1 in favor, 0 against. 18 members did not vote.

It then moved to the US Senate, where it was brought to a floor vote on April 14, 1871. Three parties were present for the vote. 44 Republicans voted in favor, 3 voted against. 0 Democrats voted in favor, 14 voted against. 1 Liberal Republican voted in favor, 2 voted against. 6 members did not vote.

In response to political violence by the Ku Klux Klan and others during the Reconstruction era following the American Civil War, Congress passed three Enforcement Acts giving the federal government broader powers to guarantee citizens' constitutional rights. The third of these acts, enacted in April 1871, gave the president the power to imprison people without a trial (known as suspending the writ of habeas corpus) and to use the federal military on domestic soil to enforce constitutional rights, among other measures.

In January 1871, Republican Senator John Scott of Pennsylvania convened a congressional committee to hear testimony from witnesses of Klan atrocities. In February, Republican Congressman Benjamin Franklin Butler of Massachusetts introduced his anti-Klan bill, intended to enforce both the Fourteenth Amendment and the Civil Rights Act of 1866. Butler's bill was narrowly defeated in the House, whereupon Republican Rep. Samuel Shellabarger, of Ohio, introduced a substitute bill, only slightly less sweeping than Butler's original. This bill brought a few holdout Republicans into line, and the bill narrowly passed the House, sailed through the Senate, and was signed into law on April 20 by President Grant.