U.S. District Judge Lynn Adelman ruled Tuesday that the law passed in 2011 is unconstitutional.
Van Hollen says, "I am disappointed with the order and continue to believe Wisconsin's law is constitutional. We will appeal."
Van Hollen is at the end of his second term in office and is not running for a third.
Republican Gov. Scott Walker has made the voter ID law one of his top priorities. He said last month that he would call the Legislature into special session to enact a law if courts ruled unfavorably.