The officer was identified as John P. Rivera, of the Hegewisch neighborhood.
Rivera and two other people, a man and woman, were sitting in their vehicle after leaving the Stout Barrel House nightclub at about 3:30 a.m. in the 700 block of North Clark Street when two men fired into the vehicle before running off.
WATCH: SUPT. JOHNSON SPEAKS AFTER OFF-DUTY COP SHOT
Rivera was shot in the chest, arm and mouth, police said. He was pronounced dead at Northwestern Memorial Hospital. A second man, also age 23 and not an officer, sustained wounds to his chest and arm, and was also taken to Northwestern in critical condition. He was sitting behind Rivera.
The shooting appears random, Chicago Police Supt. Eddie Johnson said in a Saturday afternoon press conference.
A person of interest was in custody, but police were still searching for at least two others - including the shooter.
"This appears to be an unprovoked attack against a group of individuals that went to unwind on a Friday night," said Supt. Johnson.
Nothing was taken from the people in the car, and police said the two groups didn't seem to have encountered each other earlier in the night.
Rivera and the other male officer went to the popular nightclub after finishing their Friday night shift.
The other off-duty officer and woman in the car were not hurt, police said.
Supt. Johnson said they are looking at surveillance video in the area.
"Mark my words, we will find them," Supt. Johnson warned.
Officer Rivera had served for about two years and worked at the 6th District, which is where Supt. Johnson's son works.
"He's (Rivera) the kind of officer that we want in Chicago," Johnson said. "A hard worker who loved going out on patrol and solving problems."
First responders held a procession to escort the officer's body as it was transported from the hospital to the Cook County Medical Examiner's Office.
An autopsy is scheduled for Sunday.
Shortly before 8 a.m., Chicago firefighters raised the American flag over Harrison Street outside the Cook County Medical Examiner's Office, as a procession of police vehicles drove westbound followed the ambulance carrying Rivera's body to the medical examiner's office.
Officers exited their vehicles and entered the grounds with the firefighters to hold a moment of remembrance.
"It's a very sad day, we're mourning the loss of another Chicago police officer," Fraternal Order of Police President Kevin Graham said Saturday morning outside Northwestern Memorial Hospital, adding that, despite the department's sadness, "every man from the Chicago Police Department will go out and do their job" today.
Sun-Times Media Wire contributed to this report.