This page is about districts of the Ottoman Empire; for a region in Serbia and Montenegro, see Sandžak.

Sanjak and Sandjak are the most common English transliterations of the Turkish word Sancak, which literally means "banner". They were the sub-divisions of the Ottoman provinces referred to as vilayet, eyalet or pashaluk.

Sanjaks were originally military districts of the Ottoman Empire, part of its military-feudal system. In addition to the paid professional army, the Ottoman army had corps of cavalry soldiers (called spahis or sipahi) who performed military service in return for estates granted by the sultan (larger estates were called zaim or zeamet, smaller ones timar). Spahis gathered for war according to the Sanjak in which they lived, and were led by an official called a Sanjak-beg or Sanjakbey (roughly equivalent to "district governor").

The number of Sanjaks in the Empire varied greatly. The Tanzimat reforms saw the number climb to over 400, but more usually it was around 150. Not all sanjaks were part of a vilayet, some were independent with their leaders reporting directly to the Sultan. The Sanjak of Jerusalem for instance was independent.