AFSCME Council 31 claims closing state prisons is dangerous for the public.
The union wants an injunction to stop Dwight and Tamms Correctional Center closings, along with adult transition centers across the state.
Union reps say the closures are a threat to prison employees, inmates, and the public in every region of Illinois.
The suit states nearly 5,000 inmates will be transferred and re-classified to overcrowded facilities.
They say those places can't handle violent inmates from maximum security prisons, like Tamms.
Governor Quinn wants Tamms and Dwight to close by the end of the month.
That's despite multiple hearings against the closures and the fact lawmakers passed a budget that keeps state facilities open
Quinn's office did not immediately respond for comment.