Geomorphometry, the science of land-surface analysis, is widely used in geomorphology studies to understand the sediment- and flood-producing potentials of river basins. In Turkey, fluvial geomorphology studies are generally based on qualitative information, and there have been a few geomorphometry studies regarding a single river basin or several sub-basins. In general, however, there is limited morphometric information about river basins in Turkey. This study represents the first regional geomorphometry study in Turkey, and our objective is to understand the morphometric characteristics of the Marmara Sea river basins. For this purpose, we divided the Marmara Sea basin into 632 sub-basins and identified morphometric parameters for analysis. Then, we determined the bifurcation ratio (Rb), length of overland flow (lo), drainage density (Dd), texture ratio (T), stream frequency (FS), Gravelius index (Kg), basin relief (BH), ruggedness number (Rn), hypsometric curve (Hc), and hypsometric integral (Hi) of the basins. Based on topographic maps, we produced a 10-m resolution digital elevation model with whichto define the parameters. Our study results reveal that basins located north of the Marmara Sea are tectonically younger, shallower, and longer in length, with lower drainage densities and higher hypsometric integral values than the southern basins.