Jason Van Dyke shot Laquan McDonald 16 times, seconds after arriving on the scene, saying he feared for his life as the 17-year-old had a knife.
Van Dyke was charged with first-degree murder, but has been convicted of the lesser charge.
Chicago was braced for protests. The 20 October 2014 shooting sparked outcry.
What happens now?
Van Dyke was also found guilty on all 16 counts of aggravated battery with a firearm - one count per shot fired.
The jury found him not guilty of official misconduct.
His bail was revoked and he was taken into custody following the verdict.
Van Dyke now faces a minimum of six years in prison and a maximum of over 15 years, with the possibility of probation.
The jury of eight women and four men - one African-American, seven white, one Asian-American and three Hispanic - started deliberations on Thursday afternoon.
Three other Chicago officers were charged last year with allegedly conspiring to cover up the fatal shooting, and they will be tried later this year.
How did the shooting unfold?
Mr Van Dyke was arrested in 2015 after dashcam footage appeared to show him fatally shooting Mr McDonald as he moved away from officers, contradicting official accounts.
He was the first Chicago officer to be charged with first-degree murder since 1980.
According to prosecutors, Mr McDonald was holding a knife with a 3in (7.6cm) blade when he was stopped by police.
Police said he had slashed a tyre on a patrol car, resulting in a stand-off between the teenager and officers.
Van Dyke was not among the first officers to arrive - the officer who reached the scene first told prosecutors he saw no need to use force on Mr McDonald.
Prosecutors say Van Dyke proceeded to open fire on Mr McDonald less than six seconds after he exited his patrol car, and he was the only officer to use his weapon.
Mr McDonald still had a pulse when paramedics arrived, but was declared dead in hospital.
What happened at the trial?
Van Dyke pleaded not guilty, with his defence arguing that had the teenager dropped his weapon, the officer would not have opened fire.
When he took the stand, he told the jury the teenager had been "advancing" on him with the knife, with "no expression" on his face and eyes "bugging out of his head".
Van Dyke said he continued to shoot "at the knife" after Mr McDonald was on the ground, because he "just wanted him to get rid of that knife".
During the three-week trial, the officer was still a member of the Chicago Police Department, though he was suspended without pay pending the outcome.
The trial heard from a truck driver, Rudy Barillas, who said he called police after Mr McDonald tried to stab him when he caught the teenager breaking into vehicles.
But another witness, Jose Torres, told the court that as he watched Mr Van Dyke open fire on the teenager, he thought to himself: "Why the eff are they still shooting him when he was on the ground?"
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