Thomas Postlethwaite (1731 – 4 May 1798) was an English clergyman and Cambridge fellow, Master of Trinity College, Cambridge from 1789 to 1798.Thomas Postlethwaite was the son of Richard Postlethwaite of Crooklands, near Milnthorpe, Westmorland. He went to school in St Bees before entering Trinity College, Cambridge as a sizar in 1749. Graduating BA in 1753, he became a fellow of Trinity in 1755. He was Barnaby lecturer in Mathematics in 1758.