Stephen Symonds Foster (November 17, 1809 – September 13, 1881) was a radical American abolitionist known for his dramatic and aggressive style of public speaking, and for his stance against those in the church who failed to fight slavery. His marriage to Abby Kelley brought his energetic activism to bear on women's rights. He spoke out for temperance, and agitated against any government, including his own, that would condone slavery.

Foster helped establish the New Hampshire Anti-Slavery Society, and belonged to the 'New Hampshire radicals' group within the American Anti-Slavery Society. Foster wrote anti-slavery tracts and published in 1843 a widely discussed book that met with protest and critical response: The Brotherhood of Thieves; or A True Picture of the American Church and Clergy: A Letter to Nathaniel Barney, of Nantucket. At Liberty Farm where they lived, Foster and his wife formed a link on the Underground Railroad, and helped fugitive slaves reach Canada and freedom.

Foster was born in Canterbury, New Hampshire, on November 17, 1809. His parents Sarah and Asa Foster had twelve children, Stephen was the ninth. The family attended the local Congregational church, and took part in Canterbury's anti-slavery society.

Foster apprenticed to a carpenter but left at age 22 to study to become a missionary. He went to Dartmouth College where his brother Asa had graduated and studied the classics, including Greek and Latin. Foster embraced abolitionism at this time, and in his third year invited Angelina Grimké to speak to the Young Men's Anti-Slavery Society. During his senior year, Foster was arrested and put in prison for not paying a debt of $12.14 (~$370.00 in 2024) to a local clockmaker; Foster was shocked to find that debtors were locked up with violent criminals and thieves, in common cells swarming with rats, lice, and fleas. From prison, Foster wrote a letter of protest which was published in a local paper. His friends raised bail after two weeks, but Foster's letter aroused indignation among citizens who later cleaned out the jail and then passed a law which banned imprisonment for debt. Foster redoubled his efforts in school, especially his rhetoric and public speaking courses, and graduated in 1838 third in his class.

Foster subsequently enrolled at Union Theological Seminary in New York City. There, he was forbidden by faculty to host an anti-slavery meeting he had scheduled. He was offered a scholarship if he would quit speaking of abolition, but Foster rejected this, saying he "could not be bought to hold his peace." In the spring of 1839, he left New York to take a position as traveling lecturer for the New Hampshire Anti-Slavery Society.

Foster was known to interrupt church services to denounce organized religion's complicity in slavery. In 1841, he was expelled from his Congregationalist church in Hanover, New Hampshire.

In Portland, Maine in 1842, Foster was wounded in a riot outside a meetinghouse. Pro-slavery supporters wished to prevent Foster and the radical abolitionist John Murray Spear from speaking. When the mob attacked, Foster took twenty blows to the head and had his coat torn in half. He was pulled from the crowd by women of the Portland Anti-Slavery Society who helped him escape through a back window. Spear was beaten nearly to death on the front steps of the meeting house. Local abolitionists took the men in and tended to their recuperation.

In 1843, he wrote the book The Brotherhood of Thieves; or A True Picture of the American Church and Clergy: A Letter to Nathaniel Barney, of Nantucket and in 1844 Foster's fellow abolitionist Parker Pillsbury published it through the Boston Anti-Slavery Office. The book went through twenty editions.