The Old Chicago Main Post Office is a nine-story-tall building in downtown Chicago.
The original 1922 structure was a brick-sided mail terminal building, sited just east of the main building that spans the Eisenhower Expressway as it turns into Congress Parkway. Major expansion in 1932 added a total of nine floors for more than 60 acres (24 ha), or 2.5 million square feet (230,000 m²), of floorspace. Its footprint, as initially designed, would have blocked the proposed Congress Parkway extension; as a compromise, a hole for the Parkway was reserved in the base of the Post Office and utilized twenty years later.
In 1966 the Main Chicago Post Office came to a halt when a logjam of 10 million pieces of mail clogged the system for nearly a week. With Chicago rated worst in postal deliveries, a new Main Post Office was proposed for right across Harrison Street. In 1997, the old building was vacated in favor of the new, modernized facility.
In a 500 million renovation, Gensler plans on a renovation of the space into office space, with the addition of a rooftop park and a riverwalk.