Xeer, pronounced [ħeːr], is the polycentric legal system of Somalia. Under this system, elders serve as judges and help mediate cases using precedents. It is an example of how customary law works within a stateless society and closely resembles the natural law principle. Several scholars have noted that even though Xeer may be centuries old, it has the potential to serve as the legal system of a modern, well-functioning economy.

According to one report, the Somali nation did not begin with the common use of the Somali language by the clans, but rather with the collective observance of Xeer. Xeer is thus referred to as being both the father and child of the Somali nation. An analogous phenomenon is said to have occurred among the neighboring Oromo nation, which is now under Ethiopian rule.

Under Xeer, there is no authority that dictates what the law should be. The law is instead discovered by judges as they determine the best way to resolve a dispute. As such, the Somali nation by tradition is a stateless society; that is, Somalis have never accepted the authority of any central government, their own or any other. Under Xeer law, Somalia forms a kritarchy and conforms in many respects to natural law. The lack of a central governing authority means that there is a slight variation in the interpretation of Xeer amongst different communities. The laws that are widely accepted are called xeer guud and those particular to a specific community are referred to as xeer tolnimo.