The Santa Rosa Press Democrat reports the group Stop the Casino 101 filed the lawsuit Monday to halt the casino project proposed by the Federated Indians of Graton Rancheria.
The state Legislature this month ratified an agreement that allows the tribe to begin work on the 3,000-slot machine casino. The U.S. Interior Department has 45 days to approve or reject the compact.
The plaintiffs argue the proposed casino's location is not sovereign territory and is subject to state laws barring gambling.
Tribal attorneys say the courts have already rejected those arguments.
Sonoma County supervisors may give the public a chance to weigh in on the casino's impacts next week.