János Dezső Aczél FRSC (Hungarian: [ˈɒt͡seːl]; 26 December 1924 – 1 January 2020) was a Hungarian-Canadian mathematician. He worked in functional equations and information theory. He was born in Budapest. He was the founder of the journal Aequationes Mathematicae, first published in 1968, and remained its honorary editor-in-chief.
In 1971, he was elected a fellow of the Royal Society of Canada. In 1990 he became an external member of the Hungarian Academy of Sciences.