The Thirteenth Amendment (Amendment XIII) to the United States Constitution abolished slavery and involuntary servitude, except as punishment for a crime. The amendment was passed by the Senate on April 8, 1864, by the House of Representatives on January 31, 1865, and ratified by the required 27 of the then 36 states on December 6, 1865, and proclaimed on December 18. It was the first of the three Reconstruction Amendments adopted following the American Civil War.

President Abraham Lincoln's Emancipation Proclamation, effective on January 1, 1863, declared that the enslaved in Confederate-controlled areas were free. When they escaped to Union lines or federal forces—including now-former slaves—advanced south, emancipation occurred without any compensation to the former owners. Texas was the last Confederate territory reached by the Union army. On June 19, 1865—Juneteenth—U.S. Army general Gordon Granger arrived in Galveston, Texas, to proclaim the war had ended and so had slavery (in the Confederate states). In the slave-owning areas controlled by Union forces on January 1, 1863, state action was used to abolish slavery. The exceptions were Kentucky and Delaware, where slavery was finally ended by the Thirteenth Amendment in December 1865.

In contrast to the other Reconstruction Amendments, the Thirteenth Amendment has rarely been cited in case law, but it has been used to strike down peonage and some race-based discrimination as "badges and incidents of slavery". The Thirteenth Amendment has also been invoked to empower Congress to make laws against modern forms of slavery, such as sex trafficking.

Since 1776, the Union had divided into states that allowed slavery and states that prohibited it. Slavery was implicitly recognized in the original Constitution in provisions such as Article I, Section 2, Clause 3, commonly known as the Three-Fifths Compromise, which provided that three-fifths of each state's enslaved population (“other persons”) was to be added to its free population for the purposes of apportioning seats in the United States House of Representatives, its number of Electoral votes, and direct taxes among the states. Article IV, Section 2, provided that slaves held under the laws of one state, who escaped to another state, did not become free, but remained slaves.

Though three million Confederate slaves were in fact eventually freed as a result of Lincoln's Emancipation Proclamation, their post-war status was uncertain. To ensure the abolition was beyond legal challenge, an amendment to the Constitution to that effect was initiated. On April 8, 1864, the Senate passed an amendment to abolish slavery. After one unsuccessful vote and extensive legislative maneuvering by the Lincoln administration, the House followed suit on January 31, 1865. The measure was swiftly ratified by nearly all Northern states, along with a sufficient number of border states (slave states not part of the Confederacy) up to the assassination of President Lincoln. However, the approval came via his successor, President Andrew Johnson, who encouraged the "reconstructed" Southern states of Alabama, North Carolina, and Georgia to agree, which brought the count to 27 states, leading to its adoption before the end of 1865.

Though the Amendment abolished slavery throughout the United States, some Black Americans, particularly in the South, were subjected to other forms of involuntary labor, such as under the Black Codes, as well as subjected to white supremacist violence, and selective enforcement of statutes, besides other disabilities.