The Radical Republicans were a political faction within the Republican Party originating from the party's founding in 1854—some six years before the Civil War—until the Compromise of 1877, which effectively ended Reconstruction. They called themselves "Radicals" because of their goal of immediate, complete, and permanent eradication of slavery in the United States. The Radical faction also included strong currents of nativism, anti-Catholicism, and support for the prohibition of alcoholic beverages. These policy goals and the rhetoric in their favor often made it extremely difficult for the Republican Party as a whole to avoid alienating large numbers of American voters of Irish Catholic, German, and other White ethnic backgrounds. In fact, even German-American Freethinkers and Forty-Eighters who, like Hermann Raster, otherwise sympathized with the Radical Republicans' aims, fought them tooth and nail over prohibition. They later became known as "Stalwarts".

The Radicals were opposed during the war by the Moderate Republicans (led by President Abraham Lincoln), and by the Democratic Party. Radicals led efforts after the war to establish civil rights for former slaves and fully implement emancipation. After unsuccessful measures in 1866 resulted in violence against former slaves in the former rebel states, Radicals pushed the Fourteenth Amendment for statutory protections through Congress. They opposed allowing ex-Confederate politicians and military veterans to retake political power in the Southern U.S., and emphasized equality, civil rights and voting rights for the "freedmen", i.e., former slaves who had been freed during or after the Civil War by the Emancipation Proclamation and the Thirteenth Amendment.

During the war, Radicals opposed Lincoln's initial selection of General George B. McClellan for top command of the major eastern Army of the Potomac and Lincoln's efforts in 1864 to bring seceded Southern states back into the Union as quickly and easily as possible. Lincoln later recognized McClellan as unfit and relieved him of his command. The Radicals tried passing their own Reconstruction plan through Congress in 1864. Lincoln vetoed it, as he was putting his own policy in effect through his power as military commander-in-chief. Lincoln was assassinated in April 1865. Radicals demanded for the uncompensated abolition of slavery, while Lincoln wished instead to partially emulate the British Empire's abolition of slavery by financially compensating former slave owners who had remained loyal to the Union. The Radicals, led by Thaddeus Stevens, bitterly fought Lincoln's successor, Andrew Johnson. After Johnson vetoed various congressional acts favoring citizenship for freedmen, a much harsher Reconstruction for the defeated South, and other bills he considered unconstitutional, the Radicals attempted to remove him from office through impeachment, which failed by one vote in 1868.

During the Reconstruction period, Radical Republicans supported prolabor legislation, in contrast to conservative Democrats and Liberal Republicans.

The Radicals were heavily influenced by religious ideals, and many were Protestant reformers who saw slavery as evil and the Civil War as God's punishment for slavery. The term "radical" was in common use in the anti-slavery movement before the Civil War, referring not necessarily to abolitionists, but particularly to Northern politicians strongly opposed to the Slave Power. Many and perhaps a majority had been Whigs, such as William H. Seward, a leading presidential contender in 1860 and Lincoln's Secretary of State, Thaddeus Stevens of Pennsylvania, as well as Horace Greeley, editor of the New-York Tribune, the leading Radical newspaper. There was movement in both directions: some of the pre-war Radicals (such as Seward) became less radical during the war, while some prewar moderates became Radicals. Some wartime Radicals had been Democrats before the war, often taking pro-slavery positions. They included John A. Logan of Illinois, Edwin Stanton of Ohio, Benjamin Butler of Massachusetts, and Vice President Johnson; Johnson would break with the Radicals after he became president.

The Radicals came to majority power in Congress in the elections of 1866 after several episodes of violence led many to conclude that President Johnson's weaker reconstruction policies were insufficient. These episodes included the New Orleans riot and the Memphis riots of 1866. In a pamphlet directed to black voters in 1867, the Union Republican Congressional Committee stated:

[T]he word Radical as applied to political parties and politicians ... means one who is in favor of going to the root of things; who is thoroughly in earnest; who desires that slavery should be abolished, that every disability connected therewith should be obliterated.

The Radicals were never formally organized and there was movement in and out of the group. Their most successful and systematic leader was Pennsylvania Congressman Thaddeus Stevens in the House of Representatives. The Democrats were strongly opposed to the Radicals, but they were generally a weak minority in politics until they took control of the House in the 1874 congressional elections. The "Moderate" and "Conservative" Republican factions usually opposed the Radicals, but they were not well organized. Lincoln tried to build a multi-faction coalition, including Radicals, "Conservatives," "Moderates" and War Democrats as while he was often opposed by the Radicals, he never ostracized them. Andrew Johnson was thought to be a Radical when he became president in 1865, but he soon became their leading opponent. However, Johnson could not form a cohesive support network. Finally in 1872, the Liberal Republicans, who wanted a return to classical republicanism, ran a presidential campaign and won the support of the Democratic Party for their ticket. They argued that Grant and the Radicals were corrupt and had imposed Reconstruction far too long on the South. They were overwhelmingly defeated in the 1872 election and collapsed as a movement.