"Katie and and Jamie have been dating for a while", a source told In Touch. "They're very Mission: Impossible about their romance."
The source added that Katie's ex-husband Tom Cruise's close friend and actor Jamie has secretly been spending time at Katie's apartment.
A second source said that Katie and Jamie spent the night together in mid-September at the Trump Soho Hotel. A driver arrived to take Jamie to the set of his new flick Annie but Katie turned up to take the limo instead.
"A hotel employee confirmed that Katie had stayed at the hotel with Jamie and that she was taking his car," the source said. "So the driver drove her home to her Chelsea apartment."
Rumours first began to swirl that 34-year-old Katie and 45-year-old Jamie were an item during the summer after they were spotted partying together in the Hamptons.
The Dawson's Creek actress and Django Unchained actor let their hair down at the star-studded bash in late August.
Mum-of-one Katie was photographed sipping cocktails and dancing with Jamie never far from her side.
It is likely that the pair will want to keep the nature of their relationship under wraps.
"Whoever you’re dating, don’t let anybody know," famously private Jamie once told Oprah Winfrey. "I'm very loving … when it comes to being there [for someone]."
Katie split from Tom Cruise, her husband of five years, in June 2012.
Their divorce case closed in August that year after they reached a divorce settlement in less than two weeks.
The Top Gun actor, 50, married Katie in a magnificent Italian castle ceremony in November 2006, seven months after the birth of their daughter Suri who is now seven.
Tom was previously married to Moulin Rouge star Nicole Kidman, with whom he has two children, and actress Mimi Rogers. It was Katie's first marriage.