Abolitionist, in United States history, a person who urged the immediate freeing of the slaves regardless of the Constitution, laws, or property rights. Quakers, notably Benjamin Lundy, began antislavery societies in 1815, but the abolition movement did not become nationally important until the founding in 1831 of The Liberator by William Lloyd Garrison. The Nat Turner insurrection in Virginia the same year was quickly suppressed, and resulted in uniting the South against all antislavery proposals. In the North, as well as the South, abolitionists met with denunciation and violence. A mob in Alton, Illinois, killed Elijah P. Lovejoy, an abolitionist editor. Despite the unpopularity of the abolitionists, the movement grew rapidly.

Garrison opposed action as a political party and avoided alignment with other anti-slavery groups. A faction rejecting his leadership formed the Liberty party, which ran James G. Birney for President in 1840 and 1844. It then merged with the Free Soil party, which opposed the extension of slavery, but did not try to abolish it.

Abolitionist leaders included Wendell Phillips, lawyer; John Greenleaf Whittier, poet; Henry Ward Beecher, Theodore Parker, and Theodore Weld, preachers; Angelina Grimké, who became Mrs. Weld; and her sister, Sara Grimké.

Abolitionists were outraged by passage of the Fugitive Slave Law of 1850 and became active in the "underground railroad'' to smuggle runaway slaves to freedom. Most abolitionists supported the Republican party when it was formed, but many were unhappy over the moderate proposals of its early platforms. Garrison was ready to end the abolition movement in 1865, but Phillips kept it alive until 1870, when its purposes seemed to be achieved by the passage of the 15th Amendment of the Constitution.