Thailand's Health Minister Sudarat Keyuraphan said the boy from Kanchanaburi province had died.
Earlier, a 17-year-old Vietnamese girl died. She was the 10th person to be killed by the H5N1 avian flu virus in Vietnam.
"Test results on the girl showed that she died of the H5N1 virus," said Nguyen Thi Kim Tien, director of the Ho Chi Minh City Pasteur Institute.
Details about the girl, including when she died, were not immediately available.
The World Health Organisation has not confirmed the case, Mr Robert Dietz, a spokesman for the UN health agency said on Wednesday.
Yesterday, the WHO said bird flu had killed a young man in southern Vietnam, a day after a hospital had announced the death.
The flu has devastated poultry flocks across large parts of Asia but Vietnam and Thailand are the only places where the virus has spread to people and where people have died.
Health authorities believe the victims caught the flu from contact with sick chickens but the WHO said this week it was investigating the possibility that two of the people who died in Vietnam caught it from another person.
The big fear is the bird flu virus will combine with a human flu virus and mutate and become easily passed between people who have no resistance to it.