The 47-year-old Sex and the City actress only got together with Aaron a few months ago but showed off their romance on the red carpet for the premiere of his new HBO show The Newsroom back in June.
But In Touch claims that the pair are no longer a hot item and have gone their separate ways.
"They're very different people," a source said. "She's focused on raising her daughter Gemma, and he's working on his show. They'll definitely remain friends."
Kristin adopted her baby girl last year as a single mom and has gushed about how much it has changed her life.
As well as her parenting duties, Kristin is also appearing on Broadway in Gore Vidal's The Best Man opposite Cybill Shepherd.
E! News had reported that Kristin and Aaron got together because of their shared intelligence with an insider revealing that "they have a lot to talk about."
Aaron, 51, was married once before and has a 12-year-old daughter, Roxy. He also previously dated Kristin Chenoweth.