Thomas (Tom) John Ypsilantis (June 24, 1928 in Salt Lake City, Utah – August 16, 2000, Geneva, Switzerland) was an American physicist. He was best known for the co-discovery of the antiproton in 1955, along with Owen Chamberlain, Emilio Segrè, and Clyde Wiegand. His field of research was the construction of detectors for use in particle physics. He had a long association with CERN.