In Cocke County on Wednesday, Chancellor Telford Forgety ruled a child support magistrate had acted unconstitutionally.
Magistrate Lu Ann Ballew had ordered seven-month old Messiah DeShawn Martin's given name changed to Martin.
She said Jesus Christ was the only true messiah and the name might offend.
Last year more than 700 babies were named Messiah in the US, according to the Social Security Administration.
Christianity sees Jesus as the Messiah, while Judaism uses the term to mean an anticipated saviour of the Jews.
Dictionary definitions say the word can mean anyone seen as a saviour or a liberator.
Messiah's parents had appeared before Ms Ballew in a dispute over the boy's surname - his mother Jaleesa Martin wanted him to take hers, while father Jawaan McCullough wanted him to take his.
Ms Ballew surprised them by ordering his name be changed to Martin, ruling, "Labelling this child Messiah places an undue burden on him that as a human being, he cannot fulfil.''
A Wisconsin-based organisation called Freedom From Religion filed a complaint against Ms Ballew.
At an appeal hearing on Wednesday, Mr Forgety ruled there were no legal grounds for changing the baby's given name when the parents were in agreement on it.
He also ruled Ms Ballew's decision violated the US Constitution's guarantee of the separation of church and state.
He ordered that the boy's name be changed to Messiah Deshawn McCullough.
A child in the US state of Tennessee may retain his given name Messiah, a judge has ruled, overturning a lower court's order from August.