The Breisgau S-Bahn (BSB) is a regional railway operator in the German state of Baden-Württemberg. It is a subsidiary of Freiburger Verkehrs AG (VAG), the municipal transport operator of the city of Freiburg im Breisgau, and the Südwestdeutsche Verkehrs-Aktiengesellschaft (SWEG), a transport company owned by the state of Baden-Württemberg. The company was founded in 1995, and VAG and SWEG each own 50% of its shares.
The Breisgau S-Bahn operates services over DB Netz AG owned tracks in the area around Freiburg. The services runs on short sections of the main line Rhine Valley Railway, together with the Elztalbahn and Breisachbahn branch lines.