Every public school classroom in Louisiana has been ordered to display a poster of the Ten Commandments - a move that civil liberties groups say they will challenge.
The Republican-backed measure is the first of its kind in the US, and governs all classrooms up to university level. Governor Jeff Landry signed it off on Wednesday.
Christians see the Ten Commandments as key rules from God on how to live.
The new law describes them as "foundational" to state and national governance. But opponents say the law breaks America's separation of church and state.
The first amendment to the US Constitution - known as the Establishment Clause - says that "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."
The state law requires that a poster include the sacred text in "large, easily readable font" on a poster that is 11 inches by 14 inches (28cm by 35.5cm) and that the commandments be "the central focus" of the display.
The commandments will also be shown alongside a four-paragraph "context statement" which will describe how the directives "were a prominent part of American public education for almost three centuries".
The posters must be on display in all classrooms receiving state funding by 2025 - but no state funding is being offered to pay for the posters themselves.
Similar laws have recently been proposed by other Republican-led states, including Texas, Oklahoma and Utah.
Four civil liberties groups have confirmed that they plan a legal challenge, highlighting the religious diversity of Louisiana's schools.
The law was "blatantly unconstitutional", said a joint statement from the American Civil Liberties Union, the American Civil Liberties Union of Louisiana, Americans United for Separation of Church and State, and the Freedom from Religion Foundation.
But the bill's author, Republican state lawmaker Dodie Horton, has spoken of the importance of returning a "moral code" to classrooms. She was quoted saying "it’s like hope is in the air everywhere" as the bill was rubber-stamped by the governor.
There have previously been numerous legal battles over the display of the Ten Commandments in public buildings, including courthouses and police stations as well as schools.
In 1980, in the case Stone v Graham, the US Supreme Court struck down a similar Kentucky law requiring that the document be displayed in elementary and high schools. This precedent has been cited by the groups contesting the Louisiana law.
In its ruling, the Supreme Court said the requirement that the Ten Commandments be posted "had no secular legislative purpose" and was "plainly religious in nature" - noting that the commandments made references to worshipping God.