Robert Cowdin (September 18, 1805 – July 9, 1874) was a businessman, a field officer in the Union Army during the American Civil War and a local politician in Massachusetts. Cowdin was colonel of the 1st Massachusetts Infantry and served in several major battles early in the war. He was appointed brigadier general of volunteers September 26, 1862 and he then commanded a brigade in the defenses of Washington until March 1863. He did not become a full brigadier general because his appointment expired by law March 4, 1862 when the U.S. Senate adjourned without confirming it. His grade reverted to colonel but after being relieved of command of the brigade, he returned home and saw no further service.