Wild birds were found dead at a lake near the girl’s remote village.
The disease typically spreads from birds to humans through direct contact.
The World Health Organization called for vigilance after the detection of bird flu in mammals earlier this month, but has stressed that the risk to humans is low.
The latest infection is the 58th recorded instance of bird flu in Cambodia since the country’s first human case was discovered nearly two decades ago.
Europe has been gripped by its worst-ever outbreak of bird flu since late 2021, with North and South America also experiencing severe outbreaks.
Tens of millions of domestic poultry worldwide, many with the H5N1 strain, have been culled.
The global outbreak is also responsible for the deaths of tens of thousands of wild birds.
The recent detection of the disease in a number of mammals, including foxes, otters, minks, sea lions and even grizzly bears, has sparked concern that humans could be more at risk.
Globally, there have been more than 450 fatal bird flu cases since 2003, according to the WHO.