The South Side Hospital, located at 20th and Mary Streets on Pittsburgh's South Side, was founded in 1893. The hospital building is now the site of the UPMC Mercy South Side Outpatient Center.

The hospital was originally opened inside a warehouse building on Mary Street. The hospital added an annex in 1909 and a nine-story East Wing in 1950. These older facilities were demolished in 1982 to make way for the current facility.

The original South Side Hospital and the Oliver Annex at 20th and Mary Streets in the 1920s.

On May 1, 1996, the hospital merged with the University of Pittsburgh Medical Center (UPMC) and was renamed UPMC South Side. In June 2008, UPMC announced it would close and consolidate UPMC South Side with nearby UPMC Mercy. The South Side Hospital, after over a century of service, ended it's hospital designation on June 30, 2009. UPMC South Side reopened an Outpatient Center on July 1, 2009.