Thomas Grantham (1634–1692) was an English General Baptist minister. He had access to Charles II of England, and made petitions on behalf of Baptist beliefs.
Grantham was born at Halton Holegate, near Spilsby, Lincolnshire; by trade he was a farmer. In 1644 a nonconformist congregation had been formed in the South Marsh district, between Spilsby and Boston, Lincolnshire, and one of its tenets was the rejection of sponsors in baptism. Four persons seceded from this congregation in 1651, having become Baptists. Grantham joined them, was baptised at Boston in 1653, and in 1656 was chosen their pastor. He gathered a congregation which met in private houses at Halton and elsewhere, but after considerable opposition he obtained a grant of Northolme Chapel, at Thorpe Northolme, near Wainfleet. Grantham's key convert was John Watts, a man of some property, who had received a university education, and became pastor of a baptist congregation meeting in his own house. By the efforts of Grantham and his evangelists a number of small congregations were formed in the south of Lincolnshire, holding Arminian sentiments, and so distinct from the Calvinistic Particular Baptists.
Grantham's name is not appended to the original edition (1660) of the Baptist "brief confession". But he seems to have drawn up shortly after the "narrative and complaint", which was signed by 35 General Baptists in Lincolnshire. Grantham and Joseph Wright of Westby were admitted (26 July 1660) to present the "narrative" to Charles II, with a copy of the "brief confession" and a petition for toleration. Thomas Venner's insurrection of Fifth Monarchy Men in January 1661 raised fears of Anabaptist outbreaks. Two addresses to the throne were then drawn up by Lincolnshire Baptists. The second of these was presented (23 February) by Grantham to Charles, who expressed himself as well disposed.