The Clothes Off Our Back Foundation will facilitate the online auction of the dress Knightley wore in the period romance, which is nominated for seven Academy Awards, including best picture and best costume design.
The auction begins Friday at the foundation's Web site www.clothesoffourback.org and ends March 1.
Bidding will start at $1,000.
"'Atonement' is an epic romance that has moved film goers, and garnered awards and acclaim, all over the world. As with our previous auction of the two shirts worn in 'Brokeback Mountain,' we are proud to give someone a chance to own a cherished part of a beloved film and in the process benefit the Variety charity," Focus Features CEO James Schamus said in a statement.