John Gray Foster was an American soldier. A career military officer in the United States Army and a Union general during the American Civil War, he served in North and South Carolina during the war. A reconstruction era expert in underwater demolition, he wrote a treatise on the subject in 1869. He continued with the Army after the war, using his expertise as assistant to the Chief Engineer in Washington, DC and at a post on Lake Erie.

Underwater demolition is the deliberate destruction or neutralization of man-made or natural underwater obstacles, both for military and civilian purposes.