The billboard proclaims "Keep Saturn in Saturnalia," a reference to an ancient celebration of the Roman god of agriculture. It's paid for by the Madison, Wis.-based Freedom from Religion Foundation.
Authorities say it was torched Tuesday night by two unidentified men who fled in a pickup truck.
The sign didn't burn, but its steel support beams were charred.
The atheist group has been trying since 2011 to have a privately funded Christmas banner in Pitman removed or have one added for nonbelievers.
The foundation is offering a $2,000 reward for information leading to the arrest of the suspects.