District Judge William Morvant said the amendment was flawed as drawn up by the Legislature because it had more than one purpose: banning not only gay marriage but also civil unions.
Michael Johnson, an attorney for supporters of the amendment, said he will appeal the ruling.
A gay rights group challenged the amendment on several grounds, arguing among other things that combining the question of gay marriage and the issue of civil unions in one ballot question violated state law.
The courts had rejected a similar argument before the September 18 election, saying it was premature.
Some 78 percent of those voting favored the amendment. The vote was part of a national backlash against gay marriage, which followed last year's Massachusetts Supreme Court ruling allowing gay couples to wed.
Proposals to restrict marriage to a man and a woman are on the ballot in November in 11 states: Arkansas, Georgia, Kentucky, Michigan, Mississippi, Montana, North Dakota, Ohio, Oklahoma, Oregon and Utah. Missouri voters, like those in Louisiana, overwhelmingly approved such an amendment earlier this year.
The Louisiana Legislature pushed through the proposed ban this spring. Louisiana already had a law against gay marriage, but conservatives warned that unless it was put in the state constitution, a Louisiana court could one day follow the Massachusetts example.
Christian conservatives launched a vigorous grassroots campaign to secure passage.