AbstractThis research applies a calibration-free method to estimate the significant wave height (Hs) using the amplitude of the electromagnetic signal acquired by a coherent on receive X-Band radar that operates in horizontal polarization. The ex-periments were conducted from 8th to 11 th November 2015 at the research platform FINO-3 located in the German Bight of the southern North Sea. Unlike the traditional wave disper-sion based procedure, the proposed method uses other filtering and signal processing techniques to invert the sea surface elevation from the sea clutter radar images. The compari-son of radar-derived Hs to in-situ measurements collected by a waver rider buoy resulted in a root mean square error of 0.33 m with a bias of 0.09 m and a correlation coefficient of r=0.92 . Validation of Hs estimates considering the AWAC measurements shows a RMSE of 0.24 m with a bias of 0.18 m and r=0.96 . Therefore, this calibration free methodol-ogy is very well suited to be applied to X-Band marine radars operated from offshore platforms as well as ships.