High-resolution sparker seismic profiling was carried out in the Sea of Marmara (northwest Turkey) to study the late Quaternary tectonics and sedimentation of this important active basin at the western end of the North Anatolian fault zone. A sedimentary unit interpreted as the clinoforms of a prograding delta was observed in places along the outer part of the southern shelf. The topset-foreset transitions of the latest clinoforms cluster around 100 m below present sea level; they were probably deposited during the last glacial maximum and early phase of deglaciation, between 25 and 13 k.y. B.P., when the level of the Aegean was below that of the Dardanelles sill (60–70 m below sea level at present). Therefore, the Sea of Marmara was isolated from the Aegean during the last glacial maximum, but it is possible that a fluvial overflow existed across the Dardanelles during this time, stabilizing the level of the “Marmara Lake” and allowing the delta progradation. Rivers that drain into the Sea of Marmara from the south have a much greater combined drainage basin area and sediment flux than those draining the region to the north. This may explain the much broader shelf to the south: half graben bounded by north-dipping normal faults observed within the southern shelf would tend to trap sediment sourced from the south and subdue any sea-bed morphology on the shelf.