Everything was back to normal late in the afternoon and the aquarium re-opened just before 4 p.m.
The power was out for about an hour. A Shedd Aquarium spokesperson said the outage was caused by an internal equipment malfunction.
There is a backup generator, so at no time were any of the animals in any danger.
The power failure took place around 1:30 Saturday afternoon and people were asked to temporarily leave the building until the lights came back on.
"We did ask guests to leave the building, just because some of the comfort services like restrooms that make a visit much more enjoyable were down at the time," said Andrea Smallec, Shedd Aquarium. "Everything is back up and we're business as usual."
Those who had to leave were given the option to either wait around for the power to be restored or they were given vouchers to return for free on another day.