The Riva-Cayagzi Creek (Istanbul, Black Sea) pours into the Black Sea from the coasts of Turkey. In the drainage basin of this creek, both deep and shallow drillings were undertaken, and the minimum thickness of Holocene stratigraphy was determined as 30 m. The Holocene young deposits are mainly composed of fine-grained sediments and clay, which has the most prominent thickness. The energy of water during the sedimentation process was low and stagnant in the Riva-Cayagzi Creek basin. When the marine mollusc fossils obtained from the shallow boring cores are evaluated, it can be deduced that the sea level in the study area had drawdown to -21 m in elevation. Ostracod, foraminifer, and diatom fossils determined in all cores of shallow borings are the same or similar to Mediterranean origin euryhaline forms. According to this outcome, the presence of brackish water in the study area is the result of the sea level rise that occurred at ca. 9.4 ka due to the Mediterranean origin fluctuations passing through the Dardanelles and Bosphorus straits into the Black Sea. Thus, it is understood that the sea level entered from the present Riva (Istanbul)-Black Sea coastline to at least 13 km south of the Cayagzi Creek drainage basin. The Early Holocene transgression has been defined for the first time here in the specific location of the Black Sea coast of Turkey. Accordingly, in the Riva-Cayagzi Creek basin, a lagoon sedimentation environment with low energy and stagnant water, having mainly brackish water with partial seasonal change and sea connection, was determined.