The agency was appointed without a formal review to work on the brand's US advertising.
The business had been handled by New York agency Laird & Partners, which continues to handle the bulk of Gap's business.
Laird & Partners is behind the current TV campaign starring Sarah Jessica Parker.
Gap is estimated to spend around $125m in total on advertising, with the Crispin Porter brief thought to be worth around $25m.
Crispin Porter is best known for its work on launching the BMW Mini in the US and for Ikea, which the agency resigned recently after three years following a difference of opinion with the retailer.
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