"Justin is super private about his relationships, [so] I'm not positive how they exactly met, but they've known each other for awhile, when they were seeing other people," a source tells Us. "They recently started hanging out more, going on date-like activities. They're both busy … but they're seeing where it goes."
Adds another source, the new couple have a "similar sense of humor."
Though the pair has remained low-key about their relationship, an eyewitness at a July 30 event for the Lovelace premiere tells Us that they were "inseparable."
"Justin came alone and stayed close to Amanda all night," the source says. "They weren't making out or anything like that, but they were just inseparable during the event. He hung out when she talked with others."
The A Case of You actor, 35, even showed Seyfried a bit of Twitter love on Aug. 10, writing, "Adjectives don't do justice to Amanda Seyfried's fearless + transformative perf in Lovelace."
Seyfried, 27, was previously linked to Dexter actor Desmond Harrington last summer, but the pair's reported romance fizzled a while ago, the second source confirms.
Before Harrington, Seyfried dated British actor Dominic Cooper for more than three years before splitting in 2010, a breakup that initially devastated the Les Miserables actress. Now, however, the pair still remain close friends.
"He'll always be in my life regardless of what his girlfriends or future wife think. I'm never going to be with a guy that can't deal with my friendship with him," she told ELLE magazine's August issue.