The 30 Rock actor confirmed his marriage plans by re-tweeting a congratulatory message from his friend Matthew Hiltzik.
"Congrats to my longtime friend @alecbaldwin and his wonderful bride to be @hilariathomas. Congrats on his engagement," Hiltzik wrote.
Baldwin was first linked to Thomas last summer when he moved her into his Manhattan apartment.
The Jack Donaghy actor was spotted wearing a ring on his wedding finger in January, but he strenuously denied that he was engaged at the time.
Baldwin was previously married to Kim Basinger for 12 years, having met the actress on the set of The Marrying Man.
Basinger and Baldwin are parents to teenage daughter Ireland.
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