The man died in early November in western Pailin province near the border with Thailand after being admitted to hospital with a fever, the World Health Organization and Cambodia's Ministry of Health said in a joint statement.
In the other case a three-year-old boy remains seriously ill after carrying poultry from a market in central Kampong Speu province.
"Investigations reveal the chickens and ducks had been sick and/or dead for approximately three weeks prior to the boy's onset of illness (on November 5)," the statement said.
Cambodia has reported 26 cases of H5N1 flu this year, as it battles its deadliest outbreak of the virus since it came to prominence in 2003.
Urging parents to keep children away from sick or dead poultry Health Minister Mam Bunheng said the sickness remains a "serious threat" to the health of all Cambodians.
"Children often care for domestic poultry... (and) may also have closer contact as they often treat them as pets... and play where poultry are found," he said, adding anyone with symptoms should seek urgent medical care.
Public health campaigns are being carried out in all the affected areas, the statement said, re-iterating health advice for people to wash their hands with soap after coming into contact with poultry and not to eat sick birds or carrion.
Authorities in the impoverished kingdom have been scrambling to control the outbreak, although scientists have been baffled by the sharp rise in deaths this year.
The virus has killed more than hundreds of people worldwide since a major outbreak in 2003, according to WHO statistics.
It typically spreads from birds to humans through direct contact. But experts fear it could mutate into a form easily transmissible between humans, with the potential to trigger a pandemic.