Keshub Chandra Sen (Bengali: কেশবচন্দ্র সেন; also spelled Keshab Chunder Sen; 19 November 1838 – 8 January 1884) was an Indian philosopher and social reformer who attempted to incorporate Christian theology within the framework of Hindu thought. Born a Hindu in the Bengal Presidency of British India, he became a member of the Brahmo Samaj in 1857 but established his own breakaway "Bharatvarshiya Brahmo Samaj" in 1866 while the Brahmo Samaj remained under the leadership of Debendranath Tagore (who headed the Brahmo Samaj until his death in 1905). In 1878, his followers abandoned him after the underage child marriage of his daughter which exposed his campaign against child marriage as hollow.

Keshub Chandra Sen was born on 19 November 1838 into an affluent Bengali Baidya family of Calcutta (now Kolkata). His family originally belonged to Garifa village on the banks of the river Hooghly near Naihati. His grandfather was Ramkamal Sen (1783–1844), a well-known pro-sati Hindu activist and lifelong opponent of Ram Mohan Roy. His father Peary Mohan Sen died when he was only ten years, and Sen was brought up to his uncle. As a boy, he attended the Bengali Pathshala elementary school and later attended Hindu College in 1845.

In 1855 he established an evening school for the children of working men, which continued upto 1858. In 1855, he became Secretary to the Goodwill Fraternity, a Masonic lodge associated with the Unitarian Rev. Charles Dall and a Christian missionary Rev. James Long who also helped Sen establish a "British Indian Association" in the same year. Around this time he began to be attracted to the ideas of the Brahmo Samaj.

Keshub Sen was also briefly recruited as Secretary of the Asiatic Society in 1854. For a short time thereafter Sen was also a clerk in the Bank of Bengal, but resigned his post to devote himself exclusively to literature and philosophy. On this, Professor Oman who knew him well writes, "Endowed with an emotional temperament, earnest piety, a gift of ready speech and a strong leaven of vanity, Keshub Chunder Sen found the sober, monotonous duties of a bank clerk intolerable, and very soon sought a more congenial field for the exercise of his abilities." and he formally joined the Brahma Samaj in 1859.

In 1858, left his home in Colootola and took refuge in the Jorasanko House of the Tagore family when the patriarch of the family was then away. In 1862 Sen helped found the Albert College and wrote articles for the Indian Mirror, a weekly journal of the Calcutta Brahmo Samaj in which social and moral subjects were debated.

In 1863 he wrote The Brahma Samaj Vindicated. He strongly opposed to Christianity and travelled about the country lecturing and preaching that the Brahmo Samaj was intended to revitalise Hindu religion through use of ancient Hindu sources and the authority of the Vedas. By 1865, however, Sen was convinced that only Christian doctrine could bring new life to Hindu society.

In November 1865 he was caused to leave the Brahma Samaj after "an open break with its founder Debendranath Tagore" over Christian practices in Brahmoism, and the next year (1866) with encouragement of the Unitarian preacher Charles Dall he joined another new organisation, Bharatvarshiya Brahmo Samaj, as its Secretary (President being "God"). Tagore's Brahmo Samaj then quickly purged itself of Sen's Christian teaching, and encouraged being described as Adi Brahmo Samaj to distinguish it from Sen's deliberately eponymous version.

In 1866 Sen delivered an address on "Jesus Christ, Europe and Asia", in which he proclaimed that "India would be for Christ alone who already stalks the land", and which fostered the impression that he was about to embrace Christianity.