Dane County Circuit Judge Juan Colas ruled that the law, known as Act 10, violates both the state and U.S. Constitution and is null and void, according to a report by The Associated Press. The ruling came in response to a lawsuit brought by a Madison teachers union and a union for Milwaukee city employees.
Walker spokesman Cullen Werwie told The AP he is confident the Colas decision will be overturned on appeal.
It was not clear Friday if the ruling means the law is immediately suspended.
The law revoked away nearly all collective bargaining rights from most workers and has been in effect for more than a year.
The law set into a motion several months of protests and drew a national spotlight on the state.
Walker issued the following statement in response to the court ruling: "The people of Wisconsin clearly spoke on June 5th. Now, they are ready to move on. Sadly a liberal activist judge in Dane County wants to go backwards and take away the lawmaking responsibilities of the legislature and the governor. We are confident that the state will ultimately prevail in the appeals process."
Mike Tate, chairman of the Democratic Party of Wisconsin, said, "Today is a great day for Wisconsin workers. Now, we have the opportunity to go back and do things the right way, the Wisconsin way -where everyone has a seat at the table and where we work together to find commonsense solutions."