Crest Hill police are investigating the shooting, which happened in the 900-Block of Willow Court after gathering at Crest Hill Memorial Park.
The medical examiner confirmed on Tuesday that Jamari Finch was from Joliet. He was flown to a Chicago hospital in critical condition but later died from his injuries.
Kamarion Coleman, another 18-year-old of Woodridge, died Saturday.
Two other teenagers, ages 15 and 16, are expected to recover.
Authorities have not said what led up to the shooting but said it was connected to a gathering at the park, and police responded to reports of shots fired around 11 p.m.
Police have not said whether anyone was in custody but noted that the shooting was not random.
Investigators are asking anyone with information to contact Crest Hill police at 815-741-5111. Those who wish to remain anonymous can call Crime Stoppers of Will County at 800-323-6734.