Tudor Stefan Rațiu (born March 18, 1950) is a Romanian-American mathematician who has made contributions to geometric mechanics and dynamical systems theory.

[1] His father, Mircea Ratiu, an engineer, was the younger brother of Ion Rațiu, a well-known Romanian politician, while his mother, Rodica Bucur, was a piano professor at the Conservatory of Music in Timișoara.

[1][4] After moving to the United States, he completed his Ph.D. degree at the University of California, Berkeley in 1980; his dissertation, written under the supervision of Jerrold E. Marsden, was titled Euler-Poisson Equations on Lie Algebras.

[6] In 1998 Ratiu moved to the École Polytechnique Fédérale de Lausanne, where he was a professor until 2015.

[4][7] Ratiu received a Sloan Research Fellowship in 1980,[4] and he became a fellow of the American Mathematical Society in 2012.