Both Jude and Sienna are starring on Broadway in separate productions – Jude is in "Hamlet," Sienna in "After Miss Julie."
"Sienna and Jude have met up a few times and got on really well," a source told the mag.
Jude and Sienna began dating in 2004 and ended things in 2006.
Jude reportedly broke Sienna's heart after it was revealed he cheated on her with his children's nanny.
"Sienna had her heart broken by Jude, but she was young and it was the first time she'd fallen in love," the source added. "What people don't realize is that they always remained close, so who knows where this will lead to next."