S-Bahn (/ˈɛsbɑːn/ ^ⓘ ESS-bahn, German: [ˈɛsˌbaːn] ^ⓘ; lit. 'S-train') is a type of hybrid commuter rail and rapid transit service, originating in German-speaking countries. In general, S-Bahn systems link suburbs with the city centre at moderate speeds, while continuing across the urban core over a central high-frequency corridor where multiple lines converge, where they providing a rapid means of travel across the city. The name S-Bahn derives from the German terms Schnellbahn (lit. 'rapid train'), Stadtschnellbahn (lit. 'rapid city train'), and Stadtbahn (lit. 'city train'; not to be confused with the modern Stadtbahn).

The first such network, the Berlin S-Bahn, began operation in December 1930 and inspired similar systems across Europe. The term "S-Bahn" has since become a generic term for hybrid suburban–urban rail networks, with counterparts such as the S‑tog in Denmark, S‑Trein/Train S in Belgium, Esko/S‑lines in the Czech Republic, Linee S in Italy, and SKM in Poland. The S-Bahn is also recognized as a train category in several European countries. The concept also influenced systems outside the German-speaking world, including the Réseau Express Régional (RER) in Paris and its counterparts in other cities.

There is no single, universally accepted definition of an S-Bahn system. In general, in the outer parts of a city, S-Bahn trains operate in a manner similar to conventional commuter rail, linking outlying towns with the city centre at moderate speeds. Unlike most commuter railways, however, S-Bahn services typically continue beyond the city’s main railway station, crossing the urban core on a high-frequency corridor where multiple routes converge. Stations along these central sections are spaced farther apart than on a typical rapid transit system (U-Bahn), enabling higher average speeds and providing a rapid means of travel across the city, even for passengers not travelling to or from the suburbs.

The Copenhagen S-tog, for example, operates at speeds of up to 120 kilometres per hour (75 mph), while the Rhine-Main S-Bahn operates at up to 140 km/h (87 mph). In densely populated areas, S-Bahn lines can link multiple urban centres rather than serving a single core city, as with the Rhine-Ruhr S-Bahn, which interconnects the cities and suburbs of the Ruhr region.

Many large S-Bahn systems have a dedicated underground or elevated trunk line through the central city, with multiple suburban branches feeding into it. The Berliner Stadtbahn, the main east–west axis of Berlin’s S-Bahn, is a prominent example and is also considered a tourist attraction.

Many larger S-Bahn systems have at their core a corridor of exclusive trackage that individual suburban branches feed into, creating a high frequency trunk corridor. In many cases, this central corridor is a dedicated grade-separated line in the city centre with close stop spacing and a high frequency, similar to metro systems. A good example of this is Berliner Stadtbahn in Berlin's S-Bahn, which is regarded as a tourist attraction.

Outside city centres, S-Bahn services often run on existing local or regional railway infrastructure, sometimes parallel to long-distance tracks. In some cases, new infrastructure has been purpose-built, such as the Køgebugt (Køge Bay) line in Copenhagen, constructed between 1971 and 1983, or the S-Bahn tunnel between Hamburg Central Station and Altona, completed in 1979.

S-Bahn systems are typically powered by overhead lines or a third rail; in Hamburg, both systems are used depending on the line.