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This study provides important tie-point sequences that allow better constraint on the timing and position of the marine transgression of the Arabo-Persian Gulf in southern Mesopotamia east of the ancient site of Ur in Iraq. The Mesopotamian civilisation contains the oldest city states known in the world and many other significant archaeological sites. It remains, however, an open question about the nature of the water resources that were available to these civilisations. Whilst the most northerly extent of the Arabo-Persian Gulf has been dated to ca. 6000 BP, it is much less clear how quickly the deltas of the Tigris, Euphrates and Karun infilled what is now the southern Mesopotamian Plain. Specifically, the position of the coastline is unclear at ca. 5000 BP when key city states such as Ur were founded, and also ca. 3000 BP at the start of the dynastic period, with various authors differing strongly. This is important to uncover because human populations are strongly dependent on water resources, both freshwater for drinking and washing, and freshwater and marine for naturally-occurring sources of protein from fish and wildfowl. This study addresses these concerns by the study of two sedimentary cores from locations further north than any previous studies. We have undertaken the first ever detailed molluscan analysis from such sequences in this region. This is combined with four radiocarbon dates and some indication of pollen abundance from a very sparse assemblage. Results from our study suggest that the coastline was further north than previously suggested at ca. 3000 BP, with rapid regression after this time.