The tax adds $25 to the price of every gun purchased in Cook County, and is expected to raise $60,000 in new revenue.
Cook County Board President Toni Preckwinkle says the money will help offset the costs for treating gunshot victims.
"More than a quarter of the patients that we see in trauma are gunshot victims. And it costs the taxpayers approximately $52,000 to treat each patient who enters Stroger Hospital with a gunshot wound," Preckwinkle said.
Meanwhile, a group of local gun dealers has filed a lawsuit in an attempt to stop the tax. They argue that the tax violates their constitutional rights.