The 30-year-old actress was spotted in New York, wearing a silver band on her left ring finger, reported Us magazine.
"Tommy and Amanda recently got engaged and shared the happy news with family and close friends," a source said.
Seyfried and Sadoski, 40, first met while working together on the off-Broadway show "The Way We Get By" last year and started dating when they reunited on the set of their upcoming film "The Last Word".
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