In making the announcement Monday, Guerlain described the 31-year-old actress, an Oscar winner for her roles in "Million Dollar Baby" and "Boys Don't Cry," as a "radiant and natural beauty" who does not conform to stereotypes.
"Her background of always opting for the more challenging and 'risky' roles coupled with her warm and genuine personality, a far cry from the usual Hollywood mold, were all factors that made her an obvious choice for Guerlain," the luxury French fragrance and beauty company said in a statement.
Guerlain said it has a three-year contract with Swank.
A spokeswoman for the company declined to comment Monday on the terms of the deal or the details of the fragrance.
Patti Hansen, who is married to Keith Richards of the Rolling Stones, and her daughters, Theodora and Alexandra Richards, represented Guerlain's musky fragrance Shalimar Light in 2004.
Other Oscar winners who have represented fragrances include Nicole Kidman for Chanel No. 5 and Gwyneth Paltrow for Estee Lauder's Pleasures.