They began dating last year.
His publicist Matthew Hiltzik made the engagement announcement over Twitter.
Alec, who once starred in a film called The Marrying Man, turns 54 on Tuesday, and an engagement, says Mr Hiltzik, is a "great way to celebrate".
Baldwin was previously married to Kim Basinger. They have a daughter together named Ireland.
He published a book in 2008 called A Promise To Ourselves about his personal experience dealing with divorce and his battle with Basinger over custody of their daughter.