Southampton Citybus was a bus operator which operated local services throughout the English city of Southampton. It was formed in 1898 as Southampton Corporation Transport. In 1986, as a result of deregulation an 'arms-length' limited company was formed in the name of Southampton Citybus and the coat of arms of the City of Southampton was removed, and replaced with the new branding. The company was owned by the town council until 1954, and the city council until 1993. It was then bought by its employees, in 1997 it was purchased by FirstGroup and was part of First Hampshire & Dorset until their Southampton operations ceased in February 2023.

Southampton Corporation Transport motor bus services started in 1919 as a successor to Southampton Corporation Tramways. After Southampton achieved city status in 1964 it was renamed Southampton City Transport. In 1986, as a result of deregulation it became Southampton Citybus, an arms-length company that was sold to the staff in 1993. It continued trading as Southampton Citybus until it was bought by First Bus in 1997.