Marty Beil, executive director of the Wisconsin State Employees Union, was arrested on Wednesday.
He was taken into custody shortly after a protest began in the state Capitol rotunda.
More than 300 people have been arrested in the last month, since police began cracking down on protesters who gather without the required permit.
The protesters, including singers who’ve assembled nearly every day at noon for more than two years, argue they don’t need a permit, because they’re exercising their free speech rights. The protesters oppose the policies of Gov. Walker and his fellow Republicans.
Walker said on Wednesday that protesters can easily avoid being arrested by obtaining a permit.