The stabbing occurred at 9:30 p.m. Saturday at a home on 30th Avenue and 94th Street in East Elmhurst.
When officers arrived at the scene, they found the 43-year-old off-duty officer with a stab wound to the left arm.
Police say the officer confronted a 19-year-old stranger who was trying to get into the house.
The officer and the man got into a fight, police said, and the officer was slashed.
Officials took the suspect, who was also injured, into custody at the scene. Both were taken to Booth Memorial Hospital.
The alleged attacker is still in the hospital.
Neighbors outside just want to know what led up to the stabbing.
"It's kind of unsettling," said Mika Tabanura.
Charges are pending, and the investigation continues.