Ernst Alfred Ruh (born 23 February 1936) is a Swiss mathematician, specializing in differential geometry.
Ernst Ruh received his doctorate in 1964 from Brown University under Katsumi Nomizu with thesis On the Automorphism Groups of a G-structure. He is a professor at Ohio State University and a professor of computer science at the University of Basel (1987/89). In 1990 Ruh became a full professor (professor ordinarius) of mathematics at the University of Fribourg; he was the successor of Josef Schmid. He retired in 2006 as professor emeritus.
His name is attached to the Gromov-Ruh theorem. He was an invited speaker at the International Congress of Mathematicians at Berkeley in 1986. He became a fellow of the American Mathematical Society in 2012.