Police said both incidents occurred in the uptown area, and in both incidents the suspect grabbed the rear end of a girl and then fled the scene.
In one incident, the girl was on a bicycle while in the other the suspect was jogging toward the victim.
Both incidents happened between 4 and 7 p.m. on Thursday.
Police described the suspect as a white man, 30-45 years old, with a medium to heavy-set build, red facial hair and tattoos on both arms.
If you have any information about this person, call the Park Ridge Police Investigations Unit at 847-318-5268 or call 911.