The pair were seen getting close on Saturday while attending Kate Hudson's annual Halloween bash at her Pacific Palisades home.
Phillippe, 36, and Seyfried, 25, "spent hours talking and even left the party together in Ryan's car," according to an eyewitness. "Ryan seemed very interested in Amanda and was following her around the party. She seemed flattered enough to leave the party with him."
Both stars got into the spirit of the holiday: Phillippe was dressed as Star Wars character Obi-Wan Kenobi while Seyfried wore a dog costume.
On Sunday, Seyfried was caught on video by a cameraman for x17, who asked her about the rumored romance. "You never know," the actress said coyly while getting into her car.
However, a source tells PEOPLE: "They just met up at the party. They are not dating."