Sumer (/ˈsuːmər/) is the earliest known civilization, located in the historical region of southern Mesopotamia (now south-central Iraq), emerging during the Chalcolithic and early Bronze Ages between the sixth and fifth millennium BC. Like nearby Elam, it is one of the cradles of civilization, along with Egypt, the Indus Valley, the Erligang culture of the Yellow River valley, Caral-Supe, and Mesoamerica. Living along the valleys of the Tigris and Euphrates rivers, Sumerian farmers grew an abundance of grain and other crops, a surplus of which enabled them to form urban settlements. The world's earliest known texts come from the Sumerian cities of Uruk and Jemdet Nasr, and date to between c. 3350 – c. 2500 BC, following a period of proto-writing c. 4000 – c. 2500 BC.

The term "Sumer" (Akkadian: 𒋗𒈨𒊒, romanized: šumeru) comes from the Akkadian name for the "Sumerians", the ancient non-Semitic-speaking inhabitants of southern Mesopotamia. In their inscriptions, the Sumerians called their land "Kengir", the "Country of the noble lords" (Sumerian: 𒆠𒂗𒄀, romanized: ki-en-gi(-r), lit. ''country" + "lords" + "noble''), and their language "Emegir" (Sumerian: 𒅴𒂠, romanized: eme-g̃ir or 𒅴𒄀 eme-gi₁₅).

The Akkadians, the East Semitic-speaking people who later conquered the Sumerian city-states, gave Sumer its main historical name, but the phonological development of the term šumerû is uncertain. Hebrew שִׁנְעָר Šinʿar, Egyptian Sngr, and Hittite Šanhar(a), all referring to southern Mesopotamia, could be western variants of Sumer.

Most historians have suggested that Sumer was first permanently settled between c. 5500 and c. 3300 BC by a West Asian people who spoke the Sumerian language (pointing to the names of cities, rivers, basic occupations, etc., as evidence), a non-Semitic and non-Indo-European agglutinative language isolate.

Others have suggested that the Sumerians were a North African people who migrated from the Green Sahara into the Middle East and were responsible for the spread of farming in the Middle East. However, contrary evidence strongly suggests that the first farming originated in the Fertile Crescent. Although not specifically discussing Sumerians, Lazaridis et al. 2016 have suggested a partial North African origin for some pre-Semitic cultures of the Middle East, particularly Natufians, after testing the genomes of Natufian and Pre-Pottery Neolithic culture-bearers. Craniometric analysis has also suggested an affinity between Natufians and ancient North Africans.

Some scholars associate the Sumerians with the Hurrians and Urartians, and suggest the Caucasus as their homeland. This is not generally accepted.

Based on mentions of Dilmun as the "home city of the land of Sumer" in Sumerian legends and literature, other scholars have suggested the possibility that the Sumerians originated from Dilmun, which was theorized to be the island of Bahrain in the Persian Gulf. In Sumerian mythology, Dilmun was also mentioned as the home of deities such as Enki. The status of Dilmun as the Sumerians’ ancestral homeland has not been established, but archaeologists have found evidence of civilization in Bahrain, namely the existence of Mesopotamian-style round disks.

A prehistoric people who lived in the region before the Sumerians have been termed the "Proto-Euphrateans" or "Ubaidians", and are theorized to have evolved from the Samarra culture of northern Mesopotamia. The Ubaidians, though never mentioned by the Sumerians themselves, are assumed by modern-day scholars to have been the first civilizing force in Sumer. They drained the marshes for agriculture, developed trade, and established industries, including weaving, leatherwork, metalwork, masonry, and pottery.