The actress was spotted looking "cozy" with Harrington at The Pikey in Hollywood on Wednesday, E! reports.
The pair have since been photographed walking Seyfried's dog together on Friday morning in Los Angeles.
Seyfried had a brief romance with Josh Hartnett earlier this year and was later linked to James Franco, who plays Hugh Hefner in her new movie Lovelace.
She voices a character alongside Beyoncé, Colin Farrell, Johnny Knoxville and Josh Hutcherson in the upcoming animated film Epic, which is due to be released on May 24, 2013.
Harrington plays Quinn in Dexter, which returns to Showtime on Sunday, September 30 at 9/8c.
Watch a trailer for Epic below: