Based on eyewitness reports and run-up and inundation measurements, the tsunami generated after the Kocaeli earthquake had a period shorter than a minute and arrived a few minutes after the earthquake on the northern coasts and about a minute after on the southern coasts. The sea receded first along northern and southern coastlines of Izmit Bay. This information provides valuable information on the generation mechanism of the tsunami waves, for it implies that there was a large subsidence near and/or at the shoreline, possibly caused by a stepover between two strands of the strike-slip fault system. The fairly consistent reports of approximately one-minute wave periods rule out bay oscillations as the source of the flooding and indicate that regions of subsidence may have been localized (Alpar 1999; Guneysu 1999). Tsunami waves may have also been generated by sediment slumping within the bay, in addition to tectonic subsidence. Local peaks in tsunami run-up along the northern and southern shorelines of the middle basin hint that these slumps may have generated waves near Degirmendere. The Kocaeli earthquake generated only small tsunami waves that caused no extensive damage to coastal structures. The tsunami hit close to highly populated areas, however. This is a cause of substantial concern, as there are hundreds of houses, hotels, and cafés within tens of meters of the coastline. Future earthquake hazard mitigation programs in Izmit Bay and the Sea of Marmara need to consider the hazards of tsunami waves, which may result from coseismic seafloor motion, underwater landslides, slumps, and subsidence.