In a unanimous ruling Wednesday the court affirmed the prohibition, but said it does not erode the rights of any unmarried couple to own property or craft wills, the New Orleans Times-Picayune reported.
State Judge William Morvant in East Baton Rouge Parish had ruled the amendment improperly had more than one purpose, but the state's highest court disagreed.
Both sides in the debate claimed their own victories after the ruling.
State Rep. Steve Scalise, the chief sponsor of the amendment, said marriage "will remain a sacred union between a man and a woman."
Attorney Randy Evans of the Forum for Equality was pleased the court protected the rights of unmarried couples to own property.
About 78 percent of Louisiana voters approved the constitutional amendment Sept. 18, 2004. Eleven other states adopted similar amendments last fall.