John Gray Foster (May 27, 1823 – September 2, 1874) was a career military officer in the United States Army and a Union general during the American Civil War whose most distinguished services were in North and South Carolina. A postbellum expert in underwater demolition, he wrote the definitive 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.