Themistocles M. Rassias (Greek: Θεμιστοκλής Μ. Ρασσιάς; born April 2, 1951) is a Greek mathematician, and a professor at the National Technical University of Athens (Εθνικό Μετσόβιο Πολυτεχνείο), Greece.

He serves as a member of the Editorial Board of several international mathematical journals.

He received his Ph.D. in Mathematics from the University of California at Berkeley in June 1976.

It includes Nonlinear Functional Analysis, Functional Equations, Approximation Theory, Analysis on Manifolds, Calculus of Variations, Inequalities, Metric Geometry and their Applications.

He has contributed a number of results in the stability of minimal submanifolds, in the solution of Ulam's Problem for approximate homomorphisms in Banach spaces, in the theory of isometric mappings in metric spaces and in Complex analysis (Poincaré's inequality and harmonic mappings).