The news was announced by a spokesperson for the couple, who said, "We are saddened to announce that Reese & Ryan have decided to formally separate. They remain committed to their family and we ask that you please respect their privacy and the safety of their children at this time."
Witherspoon, 30, and Phillipe, 32, met in 1997 on the birthday of the actress. They married in June of 1999, and have two children, Ava, 7, and Deacon, 3.
Witherspoon earned an Oscar for her performance in the 2006 film Walk The Line.
Phillippe is best known for his roles in the films Flags of Our Fathers, Crash, Gosford Park and 54 among others.