The baby is the son of a French man who traveled from Paris via Singapore to Phnom Penh and tested positive for the virus earlier on Sunday, the ministry said. His wife tested negative.
A Cambodian man returning from France and two other people returning from the Islamic ceremony in Malaysia also tested positive, bringing the country's tally of cases to 12, the ministry said.
Prime Minister Hun Sen has ordered citizens not to travel to Europe, the United States and Iran as part of the government's efforts to curb the spread of the virus which emerged from China late last year and has spread around the world.