Reynolds, the 34-year-old star of "The Proposal" and "X-Men Origins: Wolverine," is recently divorced from actress Scarlett Johansson.
Theron, 36, broke up with her longtime beau, actor Stuart Townsend, in January 2010.
UsMagazine.com said Reynolds and Theron ended their relationship in late July after a few months of dating.
"He only wanted something casual but she's in a rush to settle down, have kids and start a family since she's getting older," a source told the celebrity news Web site.
"[Theron] didn't take it very well when he broke it off. She knows deep down that it wouldn't have worked, but she is pretty bummed out about it all."
"They're both too busy," a friend of Theron told UsMagazine.com. "And they were hardly ever together anyway."