Roy, Rammohun (1772?–1833), political and religious thinker, was born in Radhanagar, Burdwan district, Bengal, the youngest of three children of Ramkanta Roy (d. 1803), a zamindar (landholder), and his second wife, Tarini Devi. He had an elder sister, an elder brother, Jagmohan, and a younger half-brother, Ramlochan (by his father's third wife, Rammani Devi). His date of birth is uncertain. Some authorities prefer 1774, the date given by his first biographer, Lant Carpenter, in 1833, and inscribed on his tomb in 1872; and some contemporary sources give later dates. The family was of the Rarhi Brahman caste, and worshipped Vishnu. Their hereditary title was Banerji (Sanskrit form Bandyopadhyay); the princely title Roy had been conferred on Rammohun's great-grandfather by the nawab of Bengal. Rammohun himself was apparently married three times in childhood; among Rarhi Brahmans polygyny and child marriage were common and respected practices, which he later censured. Little is known of his wives, and they did not share in his social and public life. The first died young, but the third survived him; with the second, who died in 1824, Rammohun had two sons, Radhaprasad (b. 1800) and Ramaprasad (b. 1812). He also had an adopted son, Rajaram, who accompanied him to England. (Carpenter's memoir refers to him throughout as Rammohun's son, but Carpenter was later informed that he was a foundling.)

The information from contemporary sources on Rammohun Roy's early life and education is scanty and conflicting. He is said to have been sent to Patna to study Persian and Arabic, and to Calcutta or Benares for Sanskrit; what is certain is that by 1815 he was proficient in all these languages, and in English. We also know that the family moved in 1792 to Langulpara, that in 1796 he was managing his father's estates, and that later in the same year Ramkanta divided his property among his three sons, contrary to the statement of some early biographers that Rammohun was disinherited. During the next few years Rammohun was still managing his father's property, but was also in business in Calcutta, dealing in the East India Company's stock, lending money, and investing in land; he lent large sums to some of the company's British employees. His wealth grew, while his father and his elder brother fell into debt; his father died in 1803, owing rent both to the company and to the maharaja of Burdwan. Meanwhile, Rammohun's British contacts led to employment as munshi (translator and language teacher) and as diwan (business manager); he was also intermittently employed by the company from 1803 to 1815, this employment taking him to various parts of Bengal and making him familiar with the English language and European ideas. In 1815 he and another Bengali, Krishnakant Basu, were sent to Bhutan by David Scott (1786–1831), who was becoming the company's expert on north-east frontier affairs. This mission is probably the foundation of the story, first recounted by Carpenter and much embroidered by later writers, of Rammohun's travels in Tibet—of which Bhutan was sometimes regarded as a part.

In 1815 Rammohun settled in Calcutta, where he published the bulk of his works. Most of these are in English and Bengali, and a few are in Sanskrit; they deal with religious, political, and legal matters, and also include his Bengalee Grammar in English Language (1826). In this period his European acquaintances were independent traders, journalists, and missionaries, rather than officials; he was a liberal in politics, opposed to the East India Company's monopoly but favouring independent European settlement in India. He corresponded with leading thinkers in Britain and the United States, and became known to British readers from reports in missionary journals, while his works were reviewed in British and French periodicals. He was among the pioneers of Indian journalism, founding the Sambad Kaumudi ('Moonlight of News') in Bengali and the Mir'at al-Akhbar ('Mirror of News') in Persian, both in 1822, and petitioning the government against the press regulations of 1823. In 1818–20 he contributed to the campaign against suttee (the practice whereby a wife, in principle voluntarily, burns herself with the body of her deceased husband) which culminated in a regulation forbidding the practice in 1829. Rammohun's main contribution consists of two dialogues in which he shows that suttee is not required by Sanskrit texts, as argued by its supporters, besides condemning it on humanitarian grounds. He thus joined his voice with those who rejected a feature of the Hindu tradition, while at the same time appearing as a defender of that tradition against a false interpretation. The same position can be seen in his writings against Hindu polytheism and image worship.

Some reports represented Rammohun as almost, if not quite, a Christian, others as a deist or a rationalist, but such labels do not adequately describe his religious thought. His earliest work on religion is the Tuhfat al-muwahhidin ('Gift to monotheists'), in Persian with an Arabic preface, published at Murshidabad in 1803–4. Here he argues that all religious traditions are true in that they teach belief in God as revealed in the design of the universe, but that they are all false in that they add unnecessary and conflicting beliefs to this common core. His later writings build on this foundation, but show a more accommodating attitude to religious traditions, particularly Hinduism and Christianity. In Hinduism (he was probably the first Indian writer to use this word) he accepted the Vedantic tradition founded on the Upanishads, but rejected image worship as contrary to a rational view of God. In Christianity he rejected the incarnation and the Trinity, thus allying himself with the Unitarians. During his residence in Calcutta he led a group which was variously known as the Atmiya Sabha (‘friendly society’), Brahmos (in Bengali, brahmya or brahma, ‘belonging to God’, often translated as ‘theist’ or ‘theistic’), or Hindu unitarians. After collaborating with the former Baptist missionary William Adam to form the short-lived Calcutta Unitarian Committee in 1821, he reorganized his own group as the Brahmo Sabha or Brahmo Samaj (both meaning ‘theistic society’) in 1828; under the latter name it became the most influential socio-religious modernizing movement of nineteenth-century Bengal, and inspired similar movements in other parts of India.

To support his view that 'the doctrines of the unity of God are real Hinduism', Rammohun translated selected Sanskrit texts into Bengali and English. He also compiled a selection from the gospels, The Precepts of Jesus (1820), followed by three tracts defending it against the criticism of the Baptist missionary Joshua Marshman. In a satirical series of letters published in 1823 under the pseudonym Ram Doss he proposes that trinitarian Christians and Hindu polytheists should join forces against 'this deluded sect of Unitarianism'.

In 1830 he sailed to England as ambassador of the Mughal emperor Akbar II, who conferred on him the title of raja; the East India Company refused to recognize this title, or his diplomatic status. When in Calcutta Rammohun had agreed to relinquish the title of ambassador, but he was received as such by William IV at his coronation in 1831. The purpose of the embassy was to appeal to the king for an increase in the income allowed to the emperor by the company. The actual negotiations in which Rammohun engaged were with the court of directors of the East India Company and the Board of Control (the body through which the British government controlled the company); they were partially successful. While in London Rammohun also took an active interest in Indian and British politics. He followed up his campaign against suttee by attending the meeting of the privy council on 11 July 1833, at which an appeal against the regulation of 1829 was defeated. He contributed to the discussion surrounding the renewal of the charter of the East India Company, which was completed in 1833; his submissions on this subject to the Board of Control were published in 1832 as Exposition of the Practical Operation of the Judicial and Revenue Systems of India. He watched the passage of the 1832 Reform Act with enthusiasm, and considered standing for parliament himself.

He was a striking figure, tall and richly dressed in Mughal style, with curly hair which remained black throughout his life. His last three weeks were spent in Bristol, among Unitarian friends. He died on 27 September 1833 of a fever accompanied by inflammation of the brain, as attested by J. B. Estlin, the physician who attended him and performed a post-mortem examination. His wealth at the time of his death was greatly reduced, partly through the collapse of his Calcutta agents. He was buried in the grounds of Stapleton Grove, the house in which he died. In 1843 his remains were reinterred in Arnos Vale cemetery, Bristol, and in 1844 this new grave was marked with a monument in the style of a north Indian temple, as directed by his wealthy Calcutta friend Dwarkanath Tagore (grandfather of Rabindranath Tagore), who lived in England from 1842 until his death in 1846.

Rammohun Roy's reputation has suffered from the exaggerated claims of Christians and Brahmos who saw him as a prophet of light surrounded by darkness, and later of those who saw him as 'the father of modern India'. Some of his Christian contemporaries, notably Bishop T. F. Middleton, as well as secularists of his own time and later, condemned him for providing a half-way house between traditional Hinduism and their respective creeds, while Indian nationalists have deplored his enthusiasm for the British. He still occupies a unique place between India and the West, and between ancient and modern India.