William Baldwin (March 29, 1779 – September 1, 1819) was an American physician and botanist who is today remembered for his significant contributions to botanical studies, especially Cyperaceae. He lived in Pennsylvania, Delaware, and Georgia, and served as a ship's surgeon on two voyages overseas. He published only two scientific papers, but his major contributions were in the knowledge that he imparted to other botanists in his letters to them and in the thousands of specimens that he provided for their herbaria. He wrote letters to Henry Muhlenberg, Stephen Elliott, William Darlington, Zaccheus Collins, and others. His most important collections were from Georgia, Florida, and eastern South America. When he died, he left a large herbarium that proved to be of great value, especially to Lewis David von Schweinitz, John Torrey, and Asa Gray. He had a special interest in the plant family Cyperaceae and his incomplete, unpublished manuscripts were a major source for monographs by John Torrey and Asa Gray. The historian Joseph Ewan has said that "Baldwin's treatment of a number of genera, especially in the Cyperaceae, showed penetrating observation, understanding, and diagnosis". The genus Balduina was named for him by Thomas Nuttall. Most of what we know of him is from the biography written by his friend, William Darlington, in 1843.