The couple recently announced they are expecting a baby.
"I can confirm that they are engaged," publicist Brad Cafarelli told People.com.
Alba, who has starred in "Sin City" and "Awake," was reportedly seen wearing a huge diamond ring on her left hand during the Christmas holiday.
Alba and Warren met on the set of "The Fantastic Four," where Alba was the action movie's star and Warren was a director's assistant.
They have been dating since the fall of 2004.
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