The couple's reps have confirmed the happy news to Us Weekly, who have announced the pair's engagement with the statement, 'Tommy and Amanda recently got engaged and shared the happy news with family and close friends'.
Amanda, who is known for her roles in many blockbusters including Mean Girls, Letters to Juliet and Dear John, has been seen sporting a very subtle, thin silver band on her wedding ring finger.
The happy couple met on the set of their latest film, The Last Word, which is a comedy and due to be released later this year.
After spending a lot of time together, Amanda, 30 and Thomas, 40, were first romantically linked in March - just months before the engagement announcement. However, they had previously worked together on an off-Broadway play last year, where they reportedly started a friendship.
This will be Amanda's first marriage, but the second for Thomas who is 10 years her senior. The actor had an eight-year marriage to Kimberley Hope, which ended just last year, announcing their break-up in October.
Though Amanda has never been married, she has an impressive string of celebrity boyfriends behind her including He's Just Not That Into You star Jason Long, Mamma Mia! co-star Dominic Cooper and True Blood lead Alexander Skarsgard.
Despite the couple's very private behaviour concerning their relationship so far, Amanda has previously been open about her expectations for marriage and a family.
Speaking to E!News last year she said, 'I got married so many times in my life onscreen, I don't want a white dress! I've worn so many of them.
'The fact is, that kind of stuff is less about the ceremony and more about the commitment. So it's like children. That's where it's at, right? For me. But it's different for everybody.'
She added, 'I'd like to have one in the next four or five years. And the second one can happen between 35 and 40. Or I can adopt... It's fine. I definitely want to a couple kids.'