The Phnom Penh Post said Theon Doeun of Kampong Cham died Monday night. He was the ninth known case of the disease and the eighth death since the outbreak began two months ago.
In 2011, the outbreak lasted seven months, during which there were eight cases and eight deaths, The Post reported.
Doeun is believed to have caught H5N1 after eating two infected ducks three weeks ago, the Post reported.
After Doeunn's diagnosis, more than 300 poultry were killed, the Post said.
Ly Sovann, the Ministry of Health's deputy director for communicable diseases control, said he was worried about the spread of the disease and asked residents for their help.
"I inform and appeal again to the whole population of the country, if you see sick or dead birds, you have to tell local officials and do not touch or eat those birds. Moreover, you have to tell the children to stay away from birds," he said.