The Hersek Delta which was developed by the alluvial deposits carried by the Yalakdere River is an important morphological element in the middle of the southern coasts of the İzmit Bay. In the present study, field observations, detailed bathymetric data and analyses of single-channel, high resolution seismic reflection data are used to characterise the stratal geometry and seismic stratigraphy of the Hersek Delta. The tectonised acoustic basement (unit F) is overlain by a sedimentary cover composed of 4 main seismic units (A, B, C and D/E from top to bottom). The absence of late Pleistocene aged unit C at the eastern part of the Hersek Delta implies that the waters of the Marmara Sea could not spill over the Hersek Pass during the sea level maximum (~—60 m relative to modern sea level) occurred ~25 kyr BP. During Holocene and at present, the development of the Hersek Delta continues with positive and negative accelerations under the control of tectonics and climatic changes.