The Uruk period (c. 4000/3900 to 3300/3100 BC; also known as Protoliterate period) is a period of the protohistoric Chalcolithic to Early Bronze Age in Mesopotamia and the Ancient Near East. It follows the Ubaid period and precedes the Jemdet Nasr period (although it is often considered a final stage of the Late Uruk period). In the broadest sense, this period coincides with the 4th millennium BC. Uruk culture is also used to refer to the culture originating in Lower Mesopotamia during this period.

Named after the city of Uruk in Southern Mesopotamia, this period saw a set of major innovations that lay the foundations of ancient Mesopotamian civilization. It is the period of the appearance of cities and the State (the so-called "urban revolution"), a phenomenon particularly visible in Lower Mesopotamia, notably on the site of Uruk, where excavations of the monumental center for the levels of the second half of the 4th millennium BC have revealed the existence of this culture. This phenomenon is marked by a greater specialization of activities and functions, which are accompanied by technical innovations: development of irrigated agriculture, appearance of the potter's wheel and of ceramics and bricks of standardized formats produced in large quantities, establishment of sheep farming producing wool, also on a large scale, in textile workshops, etc. The development of state institutions is accompanied by that of management instruments allowing the supervision of workers and other resources, and it is in this context that the first form of writing, 'Proto-cuneiform', appears around 3400-3300, essentially for administrative purposes.

These innovations were once thought to have originated in Uruk and southern Mesopotamia, but it has become increasingly evident that neighboring regions participated in the process and were not mere imitators. Other "proto-urban" sites and complex political entities also appeared in Susiana, southwestern Iran, northern Mesopotamia and western Syria, as well as in southeastern Anatolia. Nevertheless, Lower Mesopotamia is the most dynamic region of all, the most urbanized, the most innovative, and the most influential. It is the starting point of an 'Urukean expansion', a much-discussed long-term process seeing the implantation of outposts and colonies from southern Mesopotamia and a significant cultural impact of this region on the others. After a 'Late Uruk' phase (ca. 3500-3300/3200 BC) marking the apex of this phenomenon, after 3300/3200 BC this dynamic ceased and the Near East became culturally more fragmented.

The term "Uruk period" was coined at a conference in Baghdad in 1930, along with the preceding Ubaid period and the following Jemdet Nasr period, based on data previously collected during the excavation of a few sites (Ur, Ubayd, Kish, Girsu, Jemdet-Nasr, and also Susa in southwestern Iran), supplemented by the first discoveries made by German archaeologists in Uruk in 1929. In the following years, a thorough study conducted in Uruk, followed by the excavation of the Uruk period levels, with their objects and tablets, completed in the 1950s and 1960s, provided the basis for the definition of Uruk material culture.

The traditional chronology is very imprecise and is based on some key sondages in the Eanna quarter at Uruk. The most ancient levels of these sondages (XIX–XIII) belong to the end of the Ubaid period (Ubaid V, 4200–3900 or even 3700 BC); pottery characteristic of the Uruk period begins to appear in levels XIV/XIII. The Uruk period is traditionally divided into three main periods, subdivided in sub-phases. The first is the "Early Uruk" (levels XII to IX of the Eanna test) and then "Middle Uruk" (levels VIII to VI). From the middle of the 4th millennium BC, we gradually move towards the best-known phase, that of the "Late Uruk" (levels V and IV A and B), which lasted until around 3100 BC, or, according to more recent proposals based on carbon 14 dating, around 3300 BC.. The Cuneiform Digital Library Initiative dates the tablets of the last phase, Uruk IV, ca. from 3350 to 3200 Then comes a transitional phase, Uruk III or Jemdet Nasr, sometimes considered a final Late Uruk period. It ends ca. 3000 BC.

With the exploration of other parts of the Near East in the last decades of the 20th century, especially Northern Mesopotamia and Syria, there was a need for a new periodization. In 2001, a new chronology was proposed by the members of a colloquium at Santa Fe, based on recent excavations, especially at sites outside Mesopotamia. They consider the Uruk period to be the "Late Chalcolithic" (LC). Their LC 1 corresponds to the end of the Ubaid period and ends around 4200 BC, with the beginning of LC 2, which is the first phase of the Uruk period. They divide "Early Uruk" into two phases, with the dividing line placed around 4000 BC. Around 3800 BC, LC 3 begins, which corresponds to the "Middle Uruk" phase and continues until around 3400 BC, when it is succeeded by LC 4. It rapidly transitions to LC 5 (Late Uruk and Jemdet Nasr), which continues until 3000 BC.

Some other chronological proposals have also been put forward, such as by the ARCANE team (Associated Regional Chronologies for the Ancient Near East).

In the broad sense, the Uruk period covers the entire 4th millennium BC, since there is no radical change at the end of the Ubayd culture and the Jemdet Nasr is often undestood as a final phase of the Uruk culture.