The D1G reactor was an prototype nuclear fission reactor constructed for the U.S. Department of Energy's Office of Naval Reactors as part of the Naval Nuclear Propulsion Program.

The reactor was built by General Electric and operated by the Knolls Atomic Power Laboratory at the Kesselring Site Operation in West Milton, New York from 1962. It was used for testing components and as a training tool (Nuclear Power Training Unit). It was shut down in March 1996 and later defuelled, the pressure vessel was removed in 2002.