US Weekly was the first to report the relationship, with multiple sources confirming the news.
The pot may have been brewing for a while on this one, as Seyfried and Sadoski cozily walked the red carpet together at last year's Tony Awards in June, but both were still in relationships at the time.
Entertainment Tonight reports that Seyfried has taken a liking to actor Thomas Sadoski, who is currently co-starring with her in the upcoming film, 'The Last Word'.
The couple became friends while working together on the off-Broadway production of 'The Way We Get By' in 2015, but the relationship recently turned romantic on their current project.
The 30-year-old actress and 39-year-old actor, who have been dating since March, made it Instagram official by sharing their first photo together on the social media site after the event.
“We watched hundreds of animals find loving homes today!
She began her career as a model when she was 11, then her acting career at 15 with recurring parts on the soap operas As the World Turns and All My Children.
In 2004, Seyfried made her movie debut in the teen comedy Mean Girls.
In 2005, Seyfried played the lead character, Samantha, a role written by director Rodrigo García specifically for her, in one of the nine parts of the film Nine Lives, composed of nine short films with different themes and an ensemble cast.
Sadoski filed for divorce from casting director Kimberly Hope last October.
Sadoski and Hope married in 2007 after dating for eight years.
Representatives for the couple confirmed the news of their engagement to celebrity magazine Us Weekly.
A source had told the publication: "Tommy and Amanda recently got engaged and shared the happy news with family and close friends." Seyfried previously dated actor Justin Long and her Mamma Mia! The Newsroom star Sadoski was married to casting director Kimberly Hope for eight years before they split in summer 2015.