DuPage County Forest Preserve Chief of Police David Pederson said officers were dispatched to a 911 call near the Wood Dale Public Library just before 6:30 p.m.
Witnesses saw someone, believed to be an adult male, go swimming in the Wood Dale/Itasca Reservoir. He went underwater and did not resurface.
The Wood Dale Fire Department was the first to respond to the scene. They did not see the missing man and called in additional support, including dive teams.
Crews are now conducting a recovery mission, and the missing man's family members are on the scene, Pederson said.
Pederson said fire officials told him that the water levels are currently low, and there are not any currents in the waterway.
Pederson also said the reservoir is intended for storm and retention from the Salt Creek, and swimming is not allowed there.
Crews were not able to find the man on Tuesday night. They will resume their efforts on Wednesday morning, Pederson said.