Passengers spotted the animal Friday under an escalator and called police asking for help, the Chicago Tribune reported.
"Figured somebody was messing with us," said Chicago Police Officer Anthony Oliver.
Department of Aviation workers used a dustpan and broom to catch the animal and it was turned over to the city's animal control department, WBBM-TV, Chicago reported.
Police aren't sure how the alligator came to be at the airport, but suspect it may be the same one someone spotted being carried onto the Blue Line train.
A photo of that animal on Twitter closely matches the appearance of the one caught at the airport.
"They probably realized they couldn't take it through check point, and just let him go, but oh my God," said Tineka Walker of Universal Security.
The alligator will be released into the wild after being held and receiving medical treatment for six months. It appears its owners didn't feed it properly, WBBM said.