Modern offshore wind turbines are flexible structures with natural frequencies near the excitation frequencies related to wave and wind-induced loads. In order to obtain a reliable prediction of the structural response, the dynamic stiffness of the foundation must be evaluated accurately. This paper concerns the analysis of bucket foundations with focus on torsional motion and coupled horizontal sliding and rocking. The frequency-dependent stiffness is found by means of a three-dimensional coupled boundary-element/finite-element scheme. Comparisons are made with known analytical and numerical solutions, finding that the present boundary-element-finite-element model provides accurate results. The influence of the soil properties as well as the skirt length of the foundation is analysed, and each dynamic stiffness component is computed as function of a non-dimensional frequency.