The `30 Rock` star - who turns 54 on Tuesday (03.04.12) - popped the question to girlfriend Hilaria Thomas at the weekend and the yoga instructor said yes.
The actor`s representative told Us Weekly magazine: "Yes, it`s true! Alec proposed to Hilaria this weekend and we`re all excited for the newly engaged couple. Alec`s birthday is tomorrow. Great way to celebrate!"
Alec and Hilaria, 28, have been a couple for almost a year and first appeared in public together at several bashes in New York in May 2011.
The Hollywood star was previously married to actress Kim Bassinger but the pair divorced in acrimony in 2002 after nine years of marriage and the pair were then embroiled in a bitter custody battle over their daughter Ireland, who is now 16.
Things had been getting more and more serious between Alec and Hilaria for months, and the actor recently revealed he was considering having children with her.
He said: "Have I thought about having more kids? Oh sure, that would be great, that would be heaven, that would be fantastic."
He jokingly added: "As my friend said to me, `When you have children, typically in a second marriage, when you`re older and you get married again to a woman who would have children, you must always remember that you make sure the children attend a college where the commencement ceremonies are held in a facility with a wheelchair accessible ramp.` "