Octavius Brooks Frothingham (November 26, 1822 - November 27, 1895), was an American clergyman and author.

He was born in Boston, Massachusetts, the son of Nathaniel Langdon Frothingham (1793- 1870), a prominent Unitarian preacher, and through his mother's family he was related to Phillips Brooks. He graduated from Harvard College in 1843 and from the Divinity School in 1846. He was pastor of the North Unitarian church of Salem, Massachusetts, in 1847-1855. From 1855 to 1860 he was pastor of a new Unitarian society in Jersey City, where he gave up the Lord's Supper, thinking that it ministered to self-satisfaction; and it was as a radical Unitarian that he became pastor of another young church in New York City in 1860. Indeed in 1864 he was recognized as leader of the radicals after his reply to Dr Hedge's address to the graduating students of the Divinity School on Anti-Supernaturalism in the Pulpit.

In 1865, when he had practically given up transcendentalism, his church building was sold and his congregation began to worship in Lyric Hall under the name of the Independent Liberal Church; in 1875 they moved to the Masonic Temple , but four years later ill-health compelled Frothingham's resignation, and the church dissolved. Paralysis threatened him and he never fully recovered his health; in 1881 he returned to Boston, where he died.

To this later period of his life belongs his best literary work. While he was in New York he was for a time art critic of the Tribune. Always himself on the unpopular side and an able but thoroughly fair critic of the majority, he habitually under-estimated his own worth; he was not only an anti-slavery leader when abolition was not popular even in New England, and a radical and rationalist when it was impossible for him to stay conveniently in the Unitarian Church, but be was the first president of the National Free Religious Association (1867) and an early and ardent disciple of Darwin and Spencer.

Though always faithful to his radical views, in later years, his judgment grew more generous and catholic. He was a greater orator than a writer, and his sermons in New York were delivered to large audiences, averaging one thousand at the Masonic Temple, and were printed each week; in eloquence and in the charm of his spoken word he was probably surpassed in his day by none save George William Curtis. Personally he seemed cold and distant, partly because of his impressive appearance, and partly because of his own modesty, which made him backward in seeking friendships.

His principal published works are:

• Stories from the Life of the Teacher (1863)
• A Child's Book of Religion (1866), and other works of religious teaching for children
• several volumes of sermons
• Beliefs of Unbelievers (1876)
• The Cradle of the Christ: a Study in Primitive Christianity (1877)
• The Spirit of New Faith (1877)
• The Rising and the Setting Faith (1878), and other expositions of the new faith he preached
• Life of Theodore Parker (1874)
• Transcendentalism in New England (1876), which is largely biographical
• Gerrit Smith, a Biography (1878)
• George Ripley (1882), in the American Men of Letters series
• Memoir of William Henry Channing (1886)
• Boston Unitarianism, 1820-1850 (1890), really a biography of his father
• Recollections and Impressions, 1822-1890 (1891)