Moscow Police received a complaint just after 9:00 Wednesday morning. The sign near the corner of Jackson Street and Highway 95 on the south side of town normally reads "Millions are good without God."
Police said some time in the last 24 hours, someone covered the word "without" with black spray paint.
"The only thing I can think of is that it was offensive to someone, whoever spray painted it, so they spray painted it," said Moscow Police Department Lieutenant Dave Lehmitz.
The sign is part of an American Humanist Association campaign, and the group has signs like it all over the country.
AHA Executive Director Roy Speckhardt said they don't see the billboard as being controversial, but merely a statement of fact.
"It's saddening some people react with such antagonism to the mere statement you can be good without belief in God," said Speckhardt.
He added he wished those responsible would have opened a dialogue rather than damaged their property.
Police ask that you contact them with any information about the incident at (208) 882-COPS.
Lehmitz said they haven't determined how much the billboard is worth, but said vandalism of a billboard similar to it would fall under a misdemeanor charge.
The American Humanist Association filed a police report with the Moscow Police Department. In addition, it has alerted the billboard company they contracted with, Lamar Outdoor, who said they will assess whether or not the billboard can be cleaned. If not, the American Humanist Association will pay to have the billboard replaced.
"If we replace the billboard our next slogan might read 'Being good without God clearly not for everybody'," Speckhardt joked.