The change follows rulings by a court in Belfast and the provincial Supreme Court, UTV reported. Judges found allowing single homosexuals to adopt while excluding couples was discriminatory and could hurt children by cutting the number of adoptive parents.
Unmarried heterosexual couples will also be able to apply to become adoptive parents.
Northern Ireland, long dominated by religious conflict, has tended to be more socially conservative than the rest of the United Kingdom.
John McAllister, a member of the legislative assembly and leader of the unionist party NI21, said unmarried couples, both gay and straight, should be "commended" if they want to adopt.
"This is welcome news which brings Northern Ireland into line with our counterparts throughout the U.K.," he said.
"The interests of children have to be the number one priority."
Edwin Poots, head of the Department of Health, pursued the appeal on behalf of the government, the Belfast Telegraph reported. Poots, a member of the Democratic Unionist Party, opposes allowing homosexuals to adopt.