Lewis Tappan (May 23, 1788 – June 21, 1873) was an American abolitionist who in 1841 helped to secure freedom for the enslaved Africans aboard the Amistad. He was born in Northampton, Massachusetts, into a Calvinist household.

Tappan was also one of the founders of the American Missionary Association in 1846, which established over 100 anti-slavery Congregational churches throughout the Midwest. After the American Civil War, the association founded numerous schools and colleges to support the education of freedmen.

Contacted by Connecticut abolitionists shortly after the Amistad arrived in port, Tappan devoted significant attention to the captive Africans. He ensured the acquisition of high-quality lawyers for the captives, ultimately leading to their release after the case reached the United States Supreme Court. Alongside his brother Arthur, Tappan not only secured legal assistance and acquittal for the Africans but also successfully bolstered public support and fundraising efforts. Finally, he organized the return trip home to Africa for surviving members of the group.

Lewis Tappan was the brother of Senator Benjamin Tappan and abolitionist Arthur Tappan. His middle-class parents, Benjamin Tappan and Sarah Homes Tappan, were strict Congregationalists. Once Lewis was old enough to work, he helped his father in a dry goods store. Additionally, he entered into a silk partnership in 1826 with his brother Arthur. Lewis was acting as credit manager. On his sixteenth birthday, he explored other areas of commerce and, in 1841, he started The Mercantile Agency, the first commercial credit-rating agency in New York City. The Mercantile Agency was the precursor to Dun & Bradstreet (D&B) and modern credit-reporting services. (D&B is still in existence today.)

Convinced by Arthur to read a biography of William Wilberforce, who led the cause for abolition in Great Britain, Tappan started his quest for abolition in the United States. He is well known for his work to free the Africans from the Spanish ship Amistad.

Lewis Tappan married Susanna Aspinwall (sister of Col. Thomas Aspinwall, US consul in London) and cousin to other prominent abolitionists Samuel Aspinwall Goddard (SAG) and his nephew Rev. Samuel May of the Massachusetts Anti-Slavery Society, and whose mother was SAG's sister Mary Goddard May.

Despite his Congregationalist upbringing, Lewis Tappan became attracted to Unitarianism for intellectual and social reasons. William Ellery Channing, a Unitarian minister, became Tappan's pastor. As a peace advocate, Channing played an influential role in Tappan's decision to join the Massachusetts Peace Society. In 1827 his brother Arthur convinced him to return to a Trinitarian denomination. Tappan joined Arthur in the Congregational church. Lewis Tappan initially supported the American Colonization Society (ACS), which promoted sending freed blacks from the United States to Africa, based on the assumption that this was their homeland, regardless of where they were born.

Frustrated by the slow progress of the ACS, Tappan and a sizable nucleus of men, including his brother Arthur, Theodore Dwight Weld, Gerrit Smith, Amos A. Phelps, and James Gillespie Birney, left the ACS to join what was to become known as the "immediatist" camp, who wanted to end slavery in the United States (US). Weld gained considerable influence following the move of the Tappan brothers to this group. In December 1833, at Philadelphia, Lewis Tappan joined activists such as William Lloyd Garrison to form the American Anti-Slavery Society.