Foreigners who earn less than $2,550 per month are also barred from wedding local women, foreign ministry spokesman Koy Kuong told AFP, but the restrictions do not apply to weddings taking place overseas.
Marriages between old men and young women are "inappropriate", Koy Kuong said, and foreign men who wish to marry nationals must earn a high salary to ensure that "Cambodian women can live a decent life".
"We are preventing fake marriages and human trafficking," he said, adding that the government was aware of cases, documented by rights groups, where Cambodian women were sent into prostitution or "used as slaves" in their husband's home country.
The Cambodian foreign ministry has sent a diplomatic note to all the embassies and consulates in the country informing them of the new regulations, which came into effect on March 1.
Kek Galabru, president of local human rights group Licadho, praised the government's intention to protect Cambodian brides.
But she said the new guidelines "go against Cambodian marriage law and international law" -- specifically the Convention on the Elimination of All Forms of Discrimination Against Women.
"This is discrimination against women because they will not be allowed to marry men who are over 50... while Cambodian men can marry any foreign woman they choose," she said.
Cambodia imposed a temporary ban on foreign marriages in 2008 to prevent human trafficking, amid concern over a sharp rise in the number of brokered unions involving South Korean men and poor Cambodian women.
That ban followed an International Organisation for Migration report that said many Cambodian brides suffered abuse after moving to South Korea in marriages hastily arranged by brokers who made large profits.
The restriction was lifted about eight months later after new laws were introduced to prevent women becoming mail-order brides.