Dennis Hart Mahan (April 2, 1802 - September 16, 1871) was a noted American military theorist and professor at the United States Military Academy at West Point. He was the father of American naval theorist Rear Admiral Alfred Thayer Mahan.

A native of New York City, Mahan graduated from West Point in 1824. He started teaching at the academy soon after and was sent to Europe to study. In 1830 he was promoted to professor of civil and military engineering. As a teacher of military science, Mahan promoted defensive tactics on the battlefield. It was from him that most of the cadets who became Civil War commanders, whether Union or Confederate, learned about entrenchment and fortifications, and how to conduct siege warfare. Nowhere was his influence greater in the Civil War than at the Siege of Petersburg, where his theories affected the lives of the nearly 200,000 men in the trenches around Petersburg and Richmond.