A new, recorded roar warns museum patrons and passers-by to get down or police will be called.
The canned-roar system was installed for the NATO Summit in May.
There are no signs posted telling visitors not to scale the lions but museum officials say the bronzed statues are works of art on public display.
"They are iconic of Chicago," said the Art Institute's Erin Hogan. "They are iconic of the museum. People love the lions. We don't want someone who's on the lion to… get scared and then fall off the lion. We like to call it a gentle roar and then a 'please stay off the lions.'" The lions were given to the art institute in back in 1893.