The Les Miserables beauty and The Newsroom star are preparing to tie the knot, representatives for the couple confirm to Us Weekly, just days after Amanda was photographed wearing a simple, thin band on her left ring finger while out walking her dog Finn in New York City.
An insider tells the publication, "Tommy and Amanda recently got engaged and shared the happy news with family and close friends."
They first met in 2015, while co-starring in the Off Broadway play The Way We Get By, but their friendly relationship didn't become romantic until they reunited on the set of new movie The Last Word.
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The two have kept mum about their romance over the past year, but in March, they let pictures do all the talking, when snaps taken between scenes on the set of her new film in Pasadena, California suggested she couldn't keep her hands off Thomas.
In one revealing shot, Amanda sneakily put her left hand down the back of Sadoski's trousers as the couple walked back to her trailer during a little down time. It appeared the actor had taken the day off to visit his girlfriend at work.
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In other snaps, the actress beamed with joy, clearly very happy with her life and her new love.
Amanda also found love with her Mamma Mia! co-star, actor Dominic Cooper, and they dated from 2008 to 2013. Shortly after their break-up, she romanced actor Justin Long, but they split a year ago after more than two years together.
As for Thomas, he separated from his wife of eight years, Kimberly Hope, in October.