One teen was shot in the neck. The other was shot in the legs.
The beach was closed as police investigated. The entrance to the beach reopened at about 10 p.m. - about three hours after the shooting occurred.
The boys were in a physical altercation when an unknown offender fired shots.
When the shooting happened, the beach was crowded due to the warm weather.
"It sounded like firecrackers," said Theo Martin.
The shooting shocked many beachgoers.
"We come here every summer and nothing like this ever happens. It's usually an older crowd," said Tammi Williams-Paige. "It was shocking."
31st street Beach is closed down and CPD is clearing the existing patrons. More info on victims to follow.— Chicago Fire Media (@CFDMedia) June 12, 2017
1900 hrs 31 & Lakefront GSW Two males, unknown ages The victims were transported to Northwestern Hospital in Critical condition— Chicago Fire Media (@CFDMedia) June 12, 2017