The Mamma Mia! star was introduced to Hartnett by a mutual friend back in January (12) and the couple has been dating ever since.
A source tells Us Weekly, “Josh likes to keep things low-key, so they’ve just been hanging out.”
The acting pair was recently spotted at West Hollywood’s Chateau Marmont on a double date with Seyfried’s Gone co-star Jennifer Carpenter and her ex-husband Michael C. Hall.
Seyfried, who split from her real estate executive boyfriend Andrew Joblon at the end of last year (11), previously dated actors Ryan Phillippe and Dominic Cooper, and Hartnett has been linked to a slew of stars over the last few years, among them Kirsten Dunst, Sienna Miller, and Abbie Cornish.