The hydrodynamic responses of a floating spar wind turbine under regular and irregular waves are measured by experimental techniques and the results are validated using the industry standard time-domain modelling tool, OrcaFlex. A 1:100 scale model is built with four mooring lines and tested in a wave tank for various wave conditions. The surge, heave and pitch motions of the spar model are measured using an optical tracking system both at its centre of mass and nacelle locations. The same motions are also simulated numerically using OrcaFlex under identical wave conditions. The measured hydrodynamic responses are evaluated as Response Amplitude Operator (RAO) and compared with numerical simulations. The results show a very good agreement in RAO between the experiments and numerical simulations. Comparison with existing literature for similar spar configuration indicates that the present study better captures the non-linearities from the mooring lines and the four-point mooring configuration is found to offer significant reduction in surge motions. (C) 2012 Elsevier Ltd. All rights reserved.