Fifty-four-year-old Al Clemons, of Racine, Wisconsin, is charged with one count of felony selling a dog for the purposes of fighting.
An undercover officer from the Cook County Sheriff's office contacted Clemons. He was arrested when he met that officer in a Maywood Forest Preserve.
Police say Clemons brought along a second dog to demonstrate the pit bull's fighting skills.
The department released video of the arrest.
The pit bull has been turned over to the Animal Welfare League in Chicago Ridge.