M&C Saatchi and Saatchi & Saatchi were involved at an earlier stage. The pitch was handled by the AAR.
CHI will now take responsibility for the advertising for both of Westfield's London shopping sites: the West London site, and its Stratford City development, which is due to open in 2011.
The agency's work will run on TV, press and outdoor.
Earlier this week, CHI appointed Jonathan Burley, the Leo Burnett executive creative director, to an equivalent role after Ewan Paterson resigned to join DDB Chicago.
Westfield previously used Adam & Eve to produce its advertising. The agency, which secured the launch campaign for Westfield London in March 2008, was forced to resign the business in April last year after it picked up the £20 million John Lewis account.