Lomong fled Sudan in 1991
The United States has chosen former Sudanese refugee Lopez Lomong to carry their flag at Friday's Olympic opening ceremony in Beijing.
The move could embarrass Sudan and its ally China but Lomong said: "This is the most exciting day ever in my life."
Lomong, 23, fled his native Sudan as a six-year-old in 1991 during the height of the country's civil war.
The 1500m runner eventually reached Kenya where he lived in a refugee camp for 10 years before moving to the US.
Lomong is one of three foreign-born athletes representing the US in the event, alongside world champion Bernard Lagat, formerly of Kenya, and Leonel Manzano, who grew up in Mexico.
Lomong was chosen after a vote by the team captains of the US Olympic squad.
He added: "It is a great honour for me that my team-mates chose to vote for me. The opening ceremony is the best day and the best moment of Olympic life.
"I'm here as an ambassador of my country and I will do everything I can to represent my country well.
"This is another amazing step for me in celebrating being an American."
Last month the BBC found the first evidence that China is helping Sudan's government militarily in Darfur.
The Panorama TV programme tracked down Chinese army lorries in the Sudanese province that came from a batch exported from China to Sudan in 2005.
The BBC was also told that China was training fighter pilots who fly Chinese A5 Fantan fighter jets in Darfur, contravening a UN arms embargo on Darfur.
Some 200,000 people have died and an estimated 2.5m have been made homeless in five years of conflict in Darfur, according to international experts.
Human rights campaigners are believed to be planning to highlight the plight of Darfur in the run-up to Friday's opening ceremony.
Earlier on Wednesday the US said it would protest to China over its decision to revoke the visa of Olympic gold medallist Joey Cheek, an activist on Darfur.
Speedskater Cheek is co-founder of Team Darfur, an international coalition of athletes campaigning to draw world attention to the humanitarian crisis there.