René-Prosper Blondlot (July 3, 1849 - November 24, 1930) was a French physicist, best remembered for his mistaken identification of N rays, a phenomenon that subsequently proved to be illusory.

Born in Nancy, France, he spent most of his early years there, teaching physics at the University , being awarded three prestigious prizes of the Académie des Sciences for his experimental work on the consequences of Maxwell's theory of electromagnetism.

In order to demonstrate, in collaboration with Ernest Bichat , that a Kerr cell responds to an applied electric field in a few 10s of microseconds, he adapted the rotating-mirror method that Léon Foucault had applied to measure the speed of light. He further developed the rotating mirror to measure the speed of electricity in a conductor, photographing the sparks emitted from two conductors, one 1.8 km longer than the other and measuring the relative displacement of their images. He thus estabished that the speed of electricity in a conductor is very close to that of light.

In 1903, Blondlot announced that he had discovered N rays, a new species of radiation. The "discovery" attracted much attention over the following year until Robert W. Wood showed that the phenomena were purely subjective with no physical origin.

Blondlot lived the rest of his life in comparative obscurity in Nancy where he died.