Pattinson, 26, broke up with Stewart, 22, after she had a brief fling with Rupert Sanders, her married 41-year-old "Snow White and the Huntsman" director in July.
But The Sun said Pattinson and Stewart have since worked things out and are back together.
"They pretty much decided they couldn't live without each other," the insider said, adding, "Kristen poured her heart out to Robert and told him'' her affair with Sanders "was a one-off and a mistake."
"Rob sees it as Kristen made a really stupid mistake,'' the source said. "Rob can see how truly sorry Kristen is and has totally forgiven her. They really do love each other."