The S9 is a regional railway line of the S-Bahn Zürich on the Zürcher Verkehrsverbund (ZVV), Zürich transportation network, and is one of the network's lines connecting the cantons of Zürich and Schaffhausen in Switzerland. Between the two Swiss cantons, the line also serves two stations in Germany.

The Zürich S-Bahn system is a network of rail lines that has been incrementally expanded to cover the ZVV area, which comprises the entire canton of Zürich and portions of neighbouring cantons, with a few lines extending into or crossing the territory of southern Germany. The network is one of many commuter rail operations in German speaking countries to be described as an S-Bahn. The lines connect with services of Aargau S-Bahn to the West, Basel S-Bahn and Schaffhausen S-Bahn to the North, St. Gallen S-Bahn to the East, and Lucerne S-Bahn/Zug Stadtbahn to the South, as well as with InterCity, InterRegio and RegioExpress services at major junction stations.