The pair, who were inseparable at the premiere of Seyfried's film "Lovelace" recently, have hooked up, reported Us magazine.
"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 said.
"They recently started hanging out more, going on date-like activities. They're both busy, but they're seeing where it goes," the source added.
Seyfried has previously dated Ryan Phillippe, Dominic Cooper, Josh Hartnett and 'Dexter' actor Desmond Harrington. Long has previously dated Drew Barrymore.