Sir Harold Jeffreys, FRS[1][3] (22 April 1891 – 18 March 1989) was a British geophysicist who made significant contributions to mathematics and statistics.

One of his major contributions was on the Bayesian approach to probability (also see Jeffreys prior), as well as the idea that the Earth's planetary core was liquid.

Although the Schrödinger equation was developed two years later, Wentzel, Kramers, and Brillouin were apparently unaware of this earlier work, so Jeffreys is often neglected when credit is given for the WKB approximation.

It is only through an appendix to the third edition of Jeffreys' book Scientific Inference that we know about Mary Cartwright's method of proving that the number π is irrational.

Jeffreys, like many of his peers, staunchly opposed the concept of continental drift as put forth by Alfred Wegener and Arthur Holmes.