The Philadelphia Experiment was an alleged naval military experiment at the Philadelphia Naval Shipyard in Philadelphia, Pennsylvania, USA, sometime around October 28, 1943, in which the U.S. Navy destroyer escort USS Eldridge was to be rendered invisible (i.e.,cloaked) to human observers for a brief period of time. That is also referred to sometimes as Project Rainbow.

The story is widely regarded as a hoax,[1][2][3] while the U.S. Navy maintains that no such experiment occurred, and details of the story contradict well-established facts about the Eldridge.[4] It has nonetheless caused ripples in conspiracy theory circles, and elements of the Philadelphia Experiment are featured in other government conspiracy theories.