Ruth Jeannette Williams is an Australian-born American mathematician at the University of California, San Diego where she holds the Charles Lee Powell Chair as a Distinguished Professor of Mathematics. Her research concerns probability theory and .
Williams earned her Ph.D. from Stanford University in 1983, under the supervision of Chung Kai-lai. She was president of the Institute of Mathematical Statistics for 2011–2012.
Williams is a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences, the American Association for the Advancement of Science, the American Mathematical Society, the Institute of Mathematical Statistics, and the Institute for Operations Research and the Management Sciences.
In 2016 she and Martin I. Reiman won the John von Neumann Theory Prize "for seminal research contributions over the past several decades, to the theory and applications of stochastic networks/systems and their heavy traffic approximations".