The burgundy silk-taffeta one-shoulder gown, which was custom-made for the 21-year-old actress, will be up for bid on the eBay auction Web site from April 21 to April 30.
"This is such an amazing dress and I am really pleased to be able to donate it to Oxfam," said Knightley, nominated for a best actress Oscar for "Pride & Prejudice," in a statement Friday.
"I have seen the TV reports on the horrendous drought in East Africa and I know how desperate things have become, so I am happy to be able to do something to help."
Wang said she's "thrilled this gown can support such an important cause." The dress will be displayed at her Manhattan flagship store from April 19 until the end of the auction.
"Keira represents the essence of youth, intelligence and beauty," Wang said in a statement. "She is a young woman with a conscience."
Knightley reprises her role as Elizabeth Swann Turner in "Pirates of the Caribbean: Dead Man's Chest," set for release July 7.