The lights on some of the city's better-known landmarks were shut off for one hour.
They included the marquee at the Chicago Theatre, along with the Navy Pier Ferris wheel, and some of the city's skyscrapers.
City officials say the capitol dome in Springfield and other state offices turned off their lights.
ABC7's studio on State Street went dark as well.
ComEd saw a noticeable decline in electricity usage of about 1 percent.
More than 150 other U.S. cities are participating as well as more than 120 countries.
This is the third year the City of Chicago is participating in Earth Hour.