What is the oldest known writing system of ancient Mesopotamia?

The oldest known writing system of ancient Mesopotamia is cuneiform.

Cuneiform began in southern Mesopotamia, in the region of Sumer, during the late fourth millennium BCE. Early scribes pressed the end of a reed stylus into damp clay, creating the wedge-shaped impressions that gave the system its name. The earliest surviving tablets come from Uruk and date to around 3300 BCE.

The system likely grew from earlier accounting practices involving clay tokens and pictographic marks used to track commodities, labor and livestock. Over time, its signs became more abstract and could represent words, syllables or grammatical information. Cuneiform was therefore not one fixed alphabet: it was a flexible logo-syllabic writing tradition adapted for several languages.

Sumerian was the first language recorded with cuneiform, but Akkadian, Elamite, Hittite and other languages later used it. Cuneiform survived for more than three millennia, including royal inscriptions, legal texts, myths and administrative records. It was eventually replaced by alphabetic scripts; the latest firmly dated cuneiform tablet is from about 79–80 CE.