The Radical Republicans were a loose faction of American politicians within the Republican Party from about 1854 (before the American Civil War) until the end of Reconstruction in 1877.

The Radical Republicans had been opposed to slavery during the war, and after the war supported equal rights for freedmen (the newly freed slaves), such as measures ensuring the right to vote; passage of the Reconstruction Acts, and reduced rights for ex-Confederates. The Radicals were vigorously opposed by the Democratic Party and sometimes by more moderate Republicans as well.

The Radical Republicans opposed President Abraham Lincoln's terms for reuniting the United States during Reconstruction, which began in 1863, which they viewed as too lenient. They proposed an "ironclad oath" (which Lincoln blocked) and the Wade-Davis Bill (which Lincoln vetoed) in 1864. However the Radicals did control the Joint Committee on Reconstruction, where they demanded a more aggressive prosecution of the war, the faster end to slavery and total destruction of the Confederacy.

After the assassination of Lincoln, Vice President Andrew Johnson became president. Although he appeared at first to be a Radical, he broke with them, and the Radicals and Johnson became embroiled in a bitter struggle. After Johnson violated the Tenure of Office Act by dismissing Secretary of War Edwin M. Stanton, the House of Representatives voted to impeach him; he escaped removal from office by the Senate by a single vote in 1868, but had lost most of his power.

After the 1866 elections, the Radicals generally controlled Congress. Johnson vetoed 21 bills passed by Congress during his term, but the Radicals overrode 15 of them, including the Reconstruction Acts and Force Acts, which rewrote the election laws for the South and allowed blacks to vote. As a result of the newly empowered freedmen, the Republicans took power away from the ex-Confederates whom Johnson had appointed. The Radicals generally promoted these state Republican regimes until the last remaining three collapsed in 1877.

During the American Civil War, and later into the primary part of Reconstruction, the leading Radicals were Thaddeus Stevens in the House, and Charles Sumner in the Senate. Ulysses S. Grant was elected as a Republican in 1868; after the election he generally sided with the Radicals on Reconstruction policies (signing the Civil Rights Act of 1871 into law). The Radicals split in 1872 over Grant's reelection, and lost power after the Democrats gained control of Congress in the elections of 1874.