Bryna Rebekah Kra (born 1966) is an American mathematician who works in dynamical systems and ergodic theory, and uses dynamical methods to address problems in number theory and combinatorics. She has made contributions to the structure theory of characteristic factors for multiple ergodic averages.
She graduated with a bachelor's degree from Harvard University in 1988, and obtained her Ph.D. from Stanford University in 1995 under the guidance of Yitzhak Katznelson. She held postdoctoral positions at the Hebrew University in Jerusalem, the University of Michigan, the IHES and the Ohio State University before joining the mathematics faculty at Pennsylvania State University as an assistant professor. Since 2004 Kra has been a professor of mathematics at Northwestern University, where she was department chair from 2009 to 2012.
In 2010 Kra was awarded the Levi L. Conant Prize for her expository article 'The Green–Tao theorem on arithmetic progressions in the primes: an ergodic point of view'. In 2006 she was an invited speaker at the International Congress of Mathematicians in Madrid ('From combinatorics to ergodic theory and back again'), and was named an AMS Centennial Fellow the same year. In 2012 she became a fellow of the American Mathematical Society. From 2010-14 she was a member of the Executive Committee of the American Mathematical Society, in 2014 she was elected to the Executive Committee of the Association for Women in Mathematics, and in 2015 she was elected to the Board of Trustees of the American Mathematical Society. In 2016 she became a fellow of the American Academy of Arts and Sciences.