Children, parents and dog walkers gathered Monday to catch a peek of the reptile (estimated to be 3 1/2-4 feet long) in the North Branch of the Chicago River.
"We couldn't believe it. Our neighbors are the ones who came pounding on our door saying, 'There's an alligator! There's an alligator!" resident Angela Pollina told CBS Station WBBM.
In fact, the gator drew quite a crowd of onlookers and young explorers.
"I think it's really cool," third grader Caleb Berry told WBBM.
The first alligator sighted in the Avondale neighborhood, a 2 1/2-foot specimen, was captured by a Chicago Herpetological Society volunteer on August 6, so residents were shocked to find another unexpected visitor.
The society hopes to catch the latest alligator soon because it will not be able to survive the harsh Chicago winter.
City animal control experts believe both alligators are abandoned pets.
"They think they're doing a good thing by dumping an alligator in the river because they think it will survive here," said Bob, a Society animal wrangler. "Realistically it will not."
So that may explain why the gator seemed scared and ducked under the water when a bird approached.
Chicago's Animal Care and Control discourages people from purchasing exotic pets.
Executive director Cherie Travis told WBBM the department has a "Don't Ask, Don't Tell" policy towards owners who find they can't care for their exotic pets any more: Simply arrange for it to be dropped off at Animal Control, no questions asked.