The hostage spoke with an American accent, had a bandage on his arm and blood and dirt on his jeans. He was shown sitting in the back seat of a car, with a masked gunman next to him waving an automatic rifle, on the main highway on Baghdad's western edge where fighting took place on Friday.
The footage, taken by a cameraman from Australia's ABC television and broadcast in Australia on Saturday, was filmed on Friday. The prisoner, speaking through the car's open window, identified himself as Thomas Hamill and said he was part of a convoy that was attacked.
When asked by an ABC reporter what happened, the man said: "They attacked our convoy. That's all I'm going to say."
A Pentagon official said he was aware of the footage but could make no comments.
On Friday, the Pentagon said that several contractors and two US occupation soldiers were missing after a military fuel convoy was ambushed on the main highway west of Baghdad.
Iraq is becoming increasingly
dangerous for foreigners
Up to four civilians may be missing after Friday's convoy attack, he added.
Three Japanese, a Palestinian and a Canadian of Syrian origin are all confirmed to be kidnapped in Iraq. All of them, accept for a Japanese freelance photojournalist, are aid workers in the war-torn country. Another seven South Korean pastors were also taken hostage before being released.
Meanwhile, two German security officials have been missing for a few days while travelling from Jordan to Germany's embassy in Baghdad, but there were no signs of kidnapping, said the German Foreign Ministry on Saturday.
A spokeswoman confirmed a report by ZDF television that the two men, aged 38 and 25, had disappeared while en route to Baghdad "several days ago".
"But there are no indications they have been kidnapped," she said. She did not provide any further details.
ZDF said the two missing people were believed to be embassy guards taking part in a routine exchange of staff.
A Briton has also been reported missing. A Foreign Office official in London named the man as Gary Teeley, a British contractor. British media said Teeley, 37, was a resident in the Middle East and had been working at a US airbase.