Hudson will play "Louise", the assistant to Sarah Jessica Parker's "Carrie Bradshaw" in the film based on the hit HBO comedy series.
In July, the show's four leads, including Parker, Kim Cattrall, Kristin Davis and Cynthia Nixon, came aboard for the film after New Line Cinema agreed to finance and distribute it. Chris Noth also joined the cast to reprise his role as "Mr. Big".
The movie revolves around the four main characters' lives in Manhattan four years after the series ended.
New Line president of production Toby Emmerich, Richard Brenner and Kathy Busby are overseeing the project. Parker and John Melfi are producing. Michael Patrick King is writing and directing.
Hudson, who won a best supporting actress Oscar for Dreamgirls, recently completed production on Rowan Woods' ensemble drama Winged Creatures starring Kate Beckinsale, Forrest Whitaker, Jackie Earle Haley and Dakota Fanning.