Earlier this month, a source confirmed to ET that the two struck up a romantic relationship after filming their upcoming movie, The Last Word. The two first met in 2015 when they starred in the off-Broadway production of The Way We Get By.
Seyfried ended her two-year relationship with Justin Long in September, while Sadoski is separated and in the middle of a divorce from his ex, casting director Kimberly Hope, after eight years of marriage.
Seyfried's relationship with Thomas shouldn't come as a surprise as the Ted 2 actress has admitted in the past that she finds it difficult to separate her on-screen love life with her real life. "When you're young, it's hard not to get together with your co-star," she told W magazine in 2014. "It's not always a great idea, but the experience of making a television show or movie is so intimate and romantic. And I love acting like I'm in love!"