Thomas Ignatius Maria Forster (9 November 1789 – 2 February 1860) was an English astronomer and naturalist.
Born in London, Forster's father Thomas Furly Forster was a botanist, and follower of Jean-Jacques Rousseau. He did not have the conventional classical literary education, but he learned something of science from his uncle Benjamin Meggot Forster. The Great Comet of 1811 aroused his interest in astronomy, a science which he continued to pursue, and eight years later, on 3 July 1819, he himself discovered a new comet. He matriculated at Corpus Christi College, Cambridge, in order to study law, but soon abandoned it to study medicine, taking his degree in 1819. Two years before, he had married the daughter of Colonel Beaufoy and taken up his residence at Spa Lodge, Tunbridge Wells. After the birth of his only daughter he moved to Hartwell in Sussex, and then spent three years abroad. It was during this period that he converted to Catholicism.
After his return to England he became a fellow of the Royal Astronomical Society and helped to found a meteorological society, which had only a brief existence.
His father died in 1825, and he soon after took up his residence in Chelmsford in order to be near his daughter, who was a pupil at Newhall Convent. Here he undertook a series of researches on the influence of atmospheric conditions on diseases, and particularly on cholera. In 1833 he again went abroad, where he spent most of his remaining years, settling finally in Bruges. He continued writing during the latter part of his life, including poetry. He also composed selections for the violin. He numbered among his friends authors and scholars of his time, such as Thomas Gray, Richard Porson, Percy Bysshe Shelley, Thomas Love Peacock, William Herschel, and William Whewell. He was a vegetarian.