Czesław Ryll-Nardzewski (Polish: [ˈt͡ʂɛswav ˈrɨl narˈd͡zɛfskʲi]; 7 October 1926 – 18 September 2015) was a Polish mathematician. Born in Wilno, Second Polish Republic (now Vilnius, Lithuania), he was a student of Hugo Steinhaus. At the age of 26 he became professor at Warsaw University. In 1959, he became a professor at the Wrocław University of Technology. He was the advisor of 18 PhD theses. His main research areas are measure theory, functional analysis, foundations of mathematics and probability theory. Several theorems bear his name: the Ryll-Nardzewski fixed point theorem, the Ryll-Nardzewski theorem in model theory, and the Kuratowski and Ryll-Nardzewski measurable selection theorem.