The burgundy silk-taffeta one-shoulder gown, designed by Vera Wang for the 21-year-old actress, was bought by a bidder based in Chicago, Oxfam official Katie Abbotts said Monday.
Knightley, who was nominated for a best actress Oscar for "Pride & Prejudice," said she had wanted to support the British-based charity's efforts to provide food and clean drinking water in East Africa.
"It's brilliant that this dress has raised this much," Knightley said in a statement. "I know it will go a long way to help the people facing this food crisis in East Africa."
Wang supported the decision to put the dress, a U.S. size two, up for sale. She displayed it at her Manhattan flagship store while the online auction took place last month.
"All of us can do something to support this fantastic work, and it will make a real difference," Knightley said Monday.
Abbotts said 79 bidders had competed for the gown on the Internet auction Web site eBay.
"We know little about the winner, but from looking at their profile, we know they are from Chicago. We have contacted them by e-mail and asked them to get in touch," said Abbotts.
A casino and a used-car salesman were among other bidders and 2,500 users logged on to watch the closing minutes of the auction, she said.
Oxfam is appealing for $35 million to help 11 million people it says are at risk of hunger as a result of drought in East Africa.
Paul Smith-Lomas, the charity's East Africa director, said one of the worst droughts in a decade has devastated livestock and threatened nomadic herding communities in countries including Kenya and Somalia.
"The money Keira's dress has raised will make a vital contribution to Oxfam's work helping the 11 million people at risk," Smith-Lomas said.