The impeachment of Andrew Johnson was initiated on February 24, 1868, when the United States House of Representatives resolved to impeach Andrew Johnson, the 17th president of the United States, for "high crimes and misdemeanors", which were detailed in 11 articles of impeachment. The primary charge against Johnson was that he had violated the Tenure of Office Act, passed by Congress in March 1867 over Johnson's veto. Specifically, he had removed from office Edwin Stanton, the secretary of war, whom the act was largely designed to protect. Stanton often sided with the Radical Republican faction that passed the act, and Stanton did not have a good relationship with Johnson. Johnson attempted to replace Stanton with Brevet Major General Lorenzo Thomas. Earlier, while the Congress was not in session, Johnson had suspended Stanton and appointed General Ulysses S. Grant as secretary of war ad interim.

Johnson became the first American president to be impeached on March 23, 1868, when the House formally adopted the articles of impeachment and forwarded them to the United States Senate for adjudication. The trial in the Senate began on March 5, with Chief Justice Salmon P. Chase presiding. On May 16, the Senate did not convict Johnson on one of the articles, with the 3519 vote in favor of conviction falling one vote short of the necessary two-thirds majority. A 10-day recess was called before attempting to convict him on additional articles. On May 26, the Senate did not convict the president on two articles, both by the same margin, after which the trial was adjourned without considering the remaining eight articles of impeachment.

The impeachment and trial of Andrew Johnson had important political implications for the balance of federal legislative-executive power. It maintained the principle that Congress should not remove the president from office simply because its members disagreed with him over policy, style, and administration of the office. It also resulted in diminished presidential influence on public policy and overall governing power, fostering a system of governance which future-President Woodrow Wilson referred to in the 1880s as "Congressional Government".

Andrew Johnson (December 29, 1808 – July 31, 1875) was the 17th president of the United States, serving from 1865 to 1869. He assumed the presidency as he was vice president at the time of the assassination of Abraham Lincoln. Johnson was a Democrat who ran with Lincoln on the National Union ticket, coming to office as the Civil War concluded. He favored quick restoration of the seceded states to the Union without protection for the former slaves. This led to conflict with the Republican-dominated Congress, culminating in his impeachment by the House of Representatives in 1868. He was acquitted in the Senate by one vote.

Johnson was born into poverty and never attended school. He was apprenticed as a tailor and worked in several frontier towns before settling in Greeneville, Tennessee. He served as alderman and mayor there before being elected to the Tennessee House of Representatives in 1835. After briefly serving in the Tennessee Senate, Johnson was elected to the House of Representatives in 1843, where he served five two-year terms. He became governor of Tennessee for four years, and was elected by the legislature to the Senate in 1857. In his congressional service, he sought passage of the Homestead Bill which was enacted soon after he left his Senate seat in 1862. Southern slave states seceded to form the Confederate States of America, including Tennessee, but Johnson remained firmly with the Union. He was the only sitting senator from a Confederate state who did not resign his seat upon learning of his state's secession. In 1862, Lincoln appointed him as Military Governor of Tennessee after most of it had been retaken. In 1864, Johnson was a logical choice as running mate for Lincoln, who wished to send a message of national unity in his re-election campaign; and became vice president after a victorious election in 1864.

Johnson implemented his own form of Presidential Reconstruction, a series of proclamations directing the seceded states to hold conventions and elections to reform their civil governments. Southern states returned many of their old leaders and passed Black Codes to deprive the freedmen of many civil liberties, but Congressional Republicans refused to seat legislators from those states and advanced legislation to overrule the Southern actions. Johnson vetoed their bills, and Congressional Republicans overrode him, setting a pattern for the remainder of his presidency. Johnson opposed the Fourteenth Amendment which gave citizenship to former slaves. In 1866, he went on an unprecedented national tour promoting his executive policies, seeking to break Republican opposition. As the conflict grew between the branches of government, Congress passed the Tenure of Office Act restricting Johnson's ability to fire Cabinet officials. He persisted in trying to dismiss Secretary of War Edwin Stanton, but ended up being impeached by the House of Representatives and narrowly avoided conviction in the Senate. He did not win the 1868 Democratic presidential nomination and left office the following year.

Johnson returned to Tennessee after his presidency and gained some vindication when he was elected to the Senate in 1875, making him the only former president to serve in the Senate. He died five months into his term. Johnson's strong opposition to federally guaranteed rights for black Americans is widely criticized; he is regarded by many historians as one of the worst presidents in American history.