What's that all about?
The duo is currently filming "Live By Night" in Los Angeles, but earlier this week Ben and Sienna held hands after the cameras stopped rolling.
Ben can be seen donning a New England Patriots beanie, which is likely not his wardrobe for the 1920s crime film. Sienna, though, is done up in full costume. However, the two are attached at the hand.
That same day the hand-holding costars shared takeout food during a break in the filming.
Ben, of course, famously split with Jennifer Garner in July after 10 years of marriage. During the same month, Sienna split and called off her engagement with Tom Sturridge, who she dated for four years.
Earlier this week, OK! Magazine reported that Ben and Sienna are "really flirty on set" and that he "had Sienna in his sights for a while."
Ben, the magazine said, has remained close to Jennifer since they announced their divorce, but "he realizes their marriage is in the past and wants to focus on the present, meaning he's ready for a rebound!"
The source continued by saying, "He's not looking for anything serious, and neither is Sienna, so the timing is perfect."
Other media outlets have said there is no brewing romance between the costars.
We're trying to get a hand-le on this one.