The 40th United States Congress was a meeting of the legislative branch of the United States federal government, consisting of the United States Senate and the United States House of Representatives. It met in Washington, D.C. from March 4, 1867, to March 4, 1869, during the third and fourth years of Andrew Johnson's presidency. The apportionment of seats in the House of Representatives was based on the 1860 United States census. Both chambers had a Republican majority. In the Senate, the Republicans had the largest majority a party has ever held.

The presidency of Andrew Johnson began on April 15, 1865, when Andrew Johnson became President of the United States upon the assassination of President Abraham Lincoln, and ended on March 4, 1869. He had been Vice President of the United States for only six weeks when he succeeded to the presidency. The 17th United States president, Johnson was a member of the Democratic Party before the Civil War and had been Lincoln's 1864 running mate on the National Union ticket, which was supported by Republicans and War Democrats. Johnson took office as the Civil War came to a close, and his presidency was dominated by the aftermath of the war. As president, Johnson attempted to build his own party of Southerners and conservative Northerners, but he was unable to unite his supporters into a new party. Republican Ulysses S. Grant succeeded Johnson as president.