The seven-year-old girl was in stable condition in a Beijing hospital, the report said.
Two people who have had close contact with the child had shown no signs of being infected, Xinhua added. The girl’s parents worked in the live poultry trade, it said.
The website of China’s state radio showed a photo of the girl lying in bed, wearing a large blue face mask and with a stuffed doll next to her.
Beijing had closed all live poultry markets and banned the flying of racing pigeons, though it had not ordered a mass culling of birds since authorities had yet to find H7N9 in any animal samples, Xinhua said.
So far 11 people have died of the H7N9 bird flu strain since it was confirmed in humans for the first time last month, with 46 infections having been reported to date.
Shanghai and the eastern provinces of Zhejiang, Jiangsu and Anhui had been the only confirmed locations of infection until the Beijing case.
Xinhua said the virus could have been carried into China by migratory birds who then infected poultry.
The new virus has caused severe illness in most people affected, leading to fears that if it becomes easily transmissible, it could cause a deadly influenza pandemic, though there has been no indication of that happening.
In a bid to calm public jitters over the virus, Chinese authorities have detained a dozen people for spreading rumours about the spread of bird flu.