William Parry FRS (3 July 1934 – 20 August 2006) was an English mathematician who worked in dynamical systems, and, in particular, ergodic theory.

Although he failed the eleven-plus exam, Parry was persuaded by his mathematics teacher at Coventry Junior Technical School, specialising in metalwork and woodwork, to aim for university.

Following an MSc at the University of Liverpool, he returned to London to study at Imperial College with Yael Dowker, obtaining his PhD in 1960, with thesis Ergodic and mixing transformations.

Two years later, he gave a particularly well-received address at the Sixteenth International Congress of Mathematicians in Nice, France, and was promoted to professor.

Parry continued to contribute to the mathematical community and others' learning up until his death: he taught the University of Warwick's undergraduate course on ergodic theory as late as 2003.