The pair attended a church service in Los Angeles with Witherspoon's daughter Ava, 10, and son Deacon, 6, over the holiday weekend.
"They seemed really happy and at ease out in the open as a couple," a source tells PEOPLE. "They were all dressed up for [service] and looked festive. Jim seemed to be there to support Reese and spend time with her and her kids."
Witherspoon and Toth, who have been linked since February, recently celebrated her 34th birthday with a romantic weekend getaway in Ojai, Calif.
Witherspoon won't be seen in a movie again until December, when the James L. Brooks romantic comedy How Do You Know is released – costarring Jack Nicholson, Owen Wilson and Paul Rudd.
The actress plays a pro softball player, and reportedly spent a year doing three-hour daily workouts to prepare for the role.