Cosmas K. Zachos (Greek: Κοσμάς Ζάχος; born 1951) is a theoretical physicist. He was educated in physics (undergraduate A.B. 1974) at Princeton University, and did graduate work in theoretical physics at the California Institute of Technology (Ph.D. 1979 ) under the supervision of John Henry Schwarz. He is co-author of treatises on quantum mechanics in phase space,a Fellow of the American Physical Society, and a Fellow of the Institute of Physics.