Suppressors are not illegal in California, but they are regulated under the National Firearms Act (NFA). To own a suppressor in California, you must first obtain a permit from the Department of Justice (DOJ) and then register the suppressor with the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF).
Yes, gun suppressors are illegal in California. In California, it is a felony to possess any type of firearm suppressor, regardless of whether the device is registered or not. Violation of this law can result in up to three years in prison and a $10,000 fine.