Frigyes Riesz (January 22, 1880 – February 28, 1956) was a mathematician who was born in Győr, Hungary (Austria - Hungary) and died in Budapest, Hungary. He was rector and professor at University of Szeged. He was the older brother of the mathematician Marcel Riesz.

Riesz did some of the fundamental work in developing functional analysis and his work has had a number of important applications in physics. His work built on ideas which had been introduced by Fréchet, Lebesgue, Hilbert and others. He also made many contributions to other areas including ergodic theory and he gave an elementary proof of the mean ergodic theorem.

Riesz founded the Acta Scientiarum Mathematicarum magazine together with Alfréd Haar.

He had an uncommon method of giving lectures: he entered the lecture hall with an assistant and a docent. The docent then began reading the proper passages from Riesz's handbook and the assistant inscribed the appropriate equations on the blackboard — while Riesz himself stood aside, nodding occasionally.