John Watts de Peyster, Sr. (March 9, 1821 – May 4, 1907) was an American author on the art of war, philanthropist, and the Adjutant General of New York. He served in the New York State Militia during the Mexican–American War and American Civil War. He was one of the first military critics and noted for his histories of the Revolutionary and Civil Wars, and also published works of drama, poetry, military history, military biography and military criticism.