The wedding of a girl of 11 to a man of 41 has prompted outrage in Malaysia and calls for the minimum age for all marriages to be changed to 18.
Her Thai parents say they consented for her to become the Malaysian's third wife if he agreed she could stay at their home until she was 16.
Malaysia's government says it has no record of the marriage, which occurred in Thailand, and is investigating.
The UN children's agency said it was "shocking and unacceptable".
"It is not in the best interest of the child," said Unicef's Malaysia representative, Marianne Clark-Hattingh.
What do we know of the wedding?
Photos have emerged showing the adult groom holding the girl's hand after the marriage ceremony.
He already has two wives and six children aged between five and 18, local media reports say.
The girl's Thai family work in the north-eastern Malaysian state of Kelantan, tapping rubber from trees.
Malaysian activists say the groom is a prosperous trader while the girl's parents have lived in poverty.
What is the law in Malaysia?
While the current legal age for marriage in Malaysia is 18, Islamic sharia courts can approve Muslim marriages for those under the age of 16.
However the Malaysian government said local religious authorities had no record of this marriage.
Without permission from a religious court the marriage would be unlawful and the groom could face up to six months in jail, the ministry for women, family and community development said.
"Marrying an 11-year-old girl is like the behaviour of a child predator or paedophile," activist Syed Azmi Alhabshi told AFP news agency.
Activists say about 16,000 Malaysian girls under the age of 15 are already married, AFP reports.
Last year Malaysia passed a law on sex crimes against children but did not criminalise child marriage.
Last week the country hosted a "Girls not Brides" conference in the capital Kuala Lumpur which had ending child marriage as its theme.