The Berlin S-Bahn ("Stadtbahn", literally "city train") is a rapid transit system in and around Berlin, the capital city of Germany. It consists of 15 lines and is integrated with the mostly underground U-Bahn to form the backbone of Berlin's rapid transport system. Unlike the U-Bahn, the S-Bahn crosses the Berlin city and state border into the surrounding state of Brandenburg, mostly from the former East Berlin but today also from West Berlin to Potsdam.

Although the S- and U-Bahn are part of a unified fare system, they have different operators. The S-Bahn is operated by S-Bahn Berlin GmbH, a subsidiary of the Deutsche Bahn, whilst the U-Bahn is run by BVG, the main public transit company for the city of Berlin.