The expert for the UN Development Programme (UNDP), who has not been named, was seized in the capital Sanaa late Saturday "as part of a tribal dispute," deputy UN spokesman Eduardo del Buey told AFP.
"The tribe has made assurances regarding the safety of the staff member who reportedly remains in good health," the spokesman added.
The UN in Yemen is working with the government in a bid to end the hostage-taking. The UN "remains in contact with the staff member and representatives of the tribe," Del Buey said.
In Yemen, a tribal source told AFP that tribesmen were behind the kidnapping and that the man had been taken to the eastern province of Marib.
The source, speaking on condition of anonymity, said the tribe wants "one of their tribesmen to be released from prison.
"The kidnapping comes two months after the release of a French aid worker and her Yemeni driver and translator who were abducted in the country's restive southern region on November 22nd.
Tribes in Yemen often kidnap foreigners to put pressure on the authorities.