Aussersihler Viadukt, Railway bridge in Zürich-West, Switzerland.

The Aussersihler Viadukt is a pair of stone railway bridges in Zürich's Industriequartier, opened in 1894. The higher Wipkinger Viadukt, 834 m long, still carries trains from Zürich Hauptbahnhof toward Wipkingen and Oerlikon; the lower Lettenviadukt, 823 m, is closed to rail and serves as a foot and cycle path. The structure has 63 main openings, mostly masonry arches, the widest spanning 22.72 m.

Work on the viaduct started in 1875 and was finished in 1894, when the rail connection it carries was put into service. It was built during a period of rapid expansion in Zürich, when the western districts were being developed for industry and transport.

The name comes from the former municipality of Aussersihl, incorporated into Zürich in 1893, a year before the viaduct opened. Since 1998 the Lettenviadukt has served as a pedestrian and cycle route linking districts 5 and 6, from the Josefswiese over the Lettenbrücke to the Bahnhof Letten and along the Limmat to the Jugendkulturhaus Dynamo.

The Lettenviadukt section is open to pedestrians and cyclists, running from the Josefswiese over the Lettenbrücke to the former Letten station and on along the Limmat to the Dynamo youth centre. Its concrete-plate surface was completed in autumn 2009. The Wipkinger Viadukt carries trains and is being renovated from October 2023 to March 2025, with the line fully closed from December 2023 to December 2024.

The viaduct was bombed on 2 December 1940, damaging the structure and the overhead power lines; three railway officials were seriously injured and two more lightly. The Lettenviadukt's arches, used as warehouses and shops, had to be vacated by the end of March 2003 for the bridge's renovation.