Ensi (spelled pa.te.si, in cuneiform script, hence occasionally transliterated "patesi"; possibly derived from en si-k, "lord of the plowland"; borrowed into Akkadian as iššakkum) is a Sumerian language title designating the ruler or prince of a city-state. Originally it may have designated an independent ruler, but in later periods the title presupposed subordinance to a lugal.
For the Early Dynastic Period (about 2800–2350 BCE), the meaning of the titles en, ensí and lugal cannot be differentiated clearly: see lugal, ensi and en for details. Ensí may have originally been a designation of the ruler restricted to Lagash and Umma. The énsi was considered a representative of the city-state's patron deity. In later periods, an ensí was normally seen as subordinate to a lugal. Nevertheless, even the powerful rulers of the Second Dynasty of Lagash (c. 2100 BCE) such as Gudea were satisfied with the title ensí.
During the Third Dynasty of Ur (about 2100–2000 BC) énsi referred to the provincial governors of the kingdom. These exercised great powers in terms of government, tax revenue and jurisdiction, but they were supervised, installed, and dismissed by the lugal of Ur. Although the office could be inherited, all ensí had to be endorsed by the lugal. No independent foreign policy or warfare was allowed.
In the city-state of Assur, the hereditary ruler bore the Akkadian language version of the title ensí, while the patron deity was regarded as šarrum "king".
They held most political power in Sumerian city-states during the Uruk period (c.4100-2900 BCE).