Mr Nagin is on a 10-day trip to China and Australia
New Orleans Mayor Ray Nagin is being held in a Shanghai hotel, after a passenger on his flight from the US fell ill with suspected swine flu.
Mr Nagin's office confirmed the quarantine of the mayor, his wife Seletha, and a security guard.
They were on a 10-day trip to China and Australia aimed at promoting business to his city.
The World Health Organization says the flu strain, found in 64 countries, is most prevalent in North America.
New Orleans city officials told US media the Nagins had no symptoms and were being held as a precaution only.
They were seated during their flight near a passenger who showed "signs and symptoms of an influenza-like illness suspected to be of the H1N1 subtype".
There was no word on how long the Nagins would be held. They were to travel next to Australia.
Chinese health authorities confirmed eight cases of H1N1 swine flu on Sunday, bringing total confirmed infections on the mainland to 80.
There have been no reports of deaths from H1N1 flu in China.
Among nearly 19,000 infections worldwide, 117 people have died.
Almost 2,000 cruise ship passengers at the centre of Australia's swine flu outbreak were cleared and allowed out of quarantine in Sydney last week.