The Twilight co-stars were rumoured to have split in July following the publication of images showing Stewart in an embrace with married Snow White and the Huntsman director Rupert Sanders.
However, it has been reported that the pair have reconciled in recent weeks, after Stewart promised she did not have sex with Sanders during the affair.
A source told Us Weekly that Pattinson took Stewart back because of the strong bond between them.
"Rob weighed his options. He wouldn't even know where to start if he and Kristen were to break up for good," the insider said, while another added that the couple have a "very deep connection".
"He started thinking nobody else would understand his life," explained a friend. "His rational voice told him not to toss a three-year relationship for one infidelity."
Last week, it was suggested that Pattinson was considering marrying Stewart.