Authorities spotted the gator Friday morning and quickly wrangled the reptile, which is now in the care of the Chicago Herpetological Society.
A maintenance worker initially found the alligator. Chicago police spokesman Jose Estrada said Sunday an officer captured the reptile by putting a trash can over it.
The Chicago Herpatoligical Society's president says the animal is in "pretty bad shape" and the society is trying to nurse it back to health.
There was no word how the alligator wound up at the airport or how long it had been there.
(The Associated Press contributed to this report.)