Sir Isaac Bayley Balfour, KBE, FRS, FRSE (31 March 1853 – 30 November 1922) was a Scottish botanist. He was Regius Professor of Botany at the University of Glasgow from 1879 to 1885, Sherardian Professor of Botany at the University of Oxford from 1884 to 1888, and Professor of Botany at the University of Edinburgh from 1888 to 1922.