The Hindenburgdamm or Hindenburg Dam is an 11 km-long causeway joining the North Frisian island of Sylt to mainland Schleswig-Holstein. Its coordinates are 54°53′N 8°33′E. It was opened on 1 June 1927 and is exclusively a railway corridor. The companies that built the Hindenburgdamm, a job that took four years, were Philipp Holzmann AG of Frankfurt, working from the mainland, and Peter Fix Söhne of Duisburg working from Sylt. A train trip along the causeway takes about 10 minutes, and the time between the auto terminals at Niebüll on the mainland and Westerland on Sylt is about 30 minutes. The Hindenburgdamm is part of the railway line known as the Marschbahn, which is double-tracked along much of the route, although there as yet exists a single-tracked stretch. On the causeway is a signal box. The rail line is not electrified making the use of diesel locomotives necessary. Trains coming from origins further south like Hamburg change from an electric locomotive to a diesel locomotive at Itzehoe.

A causeway is a track, road or railway on the upper point of an embankment across "a low, or wet place, or piece of water". It can be constructed of earth, masonry, wood, or concrete. One of the earliest known wooden causeways is the Sweet Track in the Somerset Levels, England, which dates from the Neolithic age. Timber causeways may also be described as both boardwalks and bridges.