Sergei Kopeikin (born April 10, 1956) is a USSR-born physicist presently living and working in the United States, where he holds the position of Associate Professor of Physics at the University of Missouri-Columbia (UMC). He specializes in the study of gravity and general relativity, and in September 2002 he led a team which controversially claimed to be the first to have measured the speed of gravity.
Kopeikin was born in Kashin, a small town in what was then the USSR. In 1986, he obtained a Ph.D. from the Space Research Institute of the Academy of Science of the USSR in Moscow. In 1991, he obtained a Doctor of Science degree from Moscow State University.