The 30-year-old actress was spotted Wednesday holding onto the 39-year-old actor as she walked back to her trailer on set and was also seen playing with Sadoski and her dog Finn.
"Amanda and Thomas are definitely dating right now. She's no longer single," a source told Us Weekly magazine of the "Mean Girls" star.
Seyfried and Sadoski co-starred in the off-Broadway play "The Way We Get By" in 2015 and are said to have reconnected through "The Last Word." The actress will play a journalist in the new comedy, but it is unknown if Sadoski will appear in the film.
The sighting follows Seyfried's split from boyfriend Justin Long. She and the 37-year-old actor were together two years before going their separate ways in September, reportedly because they "had different schedules and different lives."
"They both tried very hard to make it work because they each wanted to eventually have a family but ultimately they weren't compatible and decided to split up," a source told People at the time. "They are still on good terms and friends."
Sadoski is known for starring in "Reasons to be Pretty" and "The House of Blue Leaves" on Broadway and played Don Keefer on HBO series "The Newsroom." The actor split from wife Kimberly Hope in 2015 after eight years of marriage.
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