al-Siq (Arabic: السيق‎) (translated: the shaft) is the main entrance to the ancient city of Petra in southern Jordan. The dim, narrow gorge (in some points no more than 3 meters wide) winds its way approximately one mile and ends at Petra's most elaborate ruin, Al Khazneh (The Treasury).

The Siq is a natural geological fault produced by tectonic forces and worn smooth by water erosion. The walls that enclose the Siq stand between 91–182 meters (300–600 feet) in height.

The entrance to the Siq contains a huge dam, reconstructed in 1963 and again in 1991, designed to bar the mouth of the Siq and reroute the waters of Wadi Musa. The dam is a fairly true reconstruction of what the Nabataeans did to control Wadi Musa between the 1st century BC and the beginning of the 1st century AD. The entrance also contains the remnants of a monumental arch, of which only the two abutments and some hewn stones of the arch itself have survived. The arch collapsed in 1896 following an earthquake, but its appearance is known from the lithographs of David Roberts.