A local magnitude scale is derived for northwestern Turkey, using data collected by a temporary network installed by the German Task Force for Earthquakes after the 1999 Izmit earthquake (Mw 7.4) and the permanent Sapanca-Bolu network. We computed Wood–Anderson seismograms for over 5353 arrivals at 27 three-component stations, from 530 earthquakes. The hypocentral distances considered range from 5 to 140 km, with the best represented range being from 5 to 70 km. We inverted the measured amplitudes following both the nonparametric Richter's (1958) and parametric Bakun and Joyner (1984) approaches. These methods yield consistent magnitude values and station corrections. However, the calculated nonparametric distance correction, logA0, implies that ground-motion attenuation is higher than what is accounted for by the equation calibrated in central California. In the range 5–62 km, the best fit is provided by -logA0 = log(R/17) + 0.00960(R - 17) + 2. This equation is obtained constraining the geometrical spreading parameter n to 1. The calculated value of k = 0.00960 confirms that in this range of distance seismic waves could propagate through a low-Q volume. Station corrections, which allow for a significant reduction of ML residuals, range between ±0.5 magnitude units, suggesting a strong influence of local site effects on the amplitude of ground motion. In accord with the obtained attenuation and station corrections, the magnitudes of the considered events range from 0.4 to 4.8.