John Ellis Wool (February 20, 1784 – November 10, 1869) was an officer in the United States Army during three consecutive U.S. wars: the War of 1812, the Mexican–American War and the American Civil War.

By the time of the Mexican-American War, he is widely considered one of the most capable officers in the army and a superb organizer.

He is one of the four general officers of the United States Army in 1861, and is the one who has the most Civil War service.

When the war begins, Wool, age seventy-seven and a brigadier general for twenty years, commands the Department of the East.

He is the oldest general on either side of the war.