Ioan Mackenzie James (born 23 May 1928 Croydon, Surrey, England) is a British mathematician working in the field of topology particularly in homotopy theory. He is a fellow of the Royal Society.
He received his D.Phil. from the University of Oxford in 1953. His thesis, titled Some problems in algebraic topology, was written under the direction of J. H. C. Whitehead.
He held the Savilian Chair of Geometry at the University of Oxford from 1970 to 1995. He is now a professor emeritus.
In 1978 the London Mathematical Society awarded him the Senior Whitehead Prize, which was established in honour of his PhD supervisor, Whitehead.