The structural stability of offshore wind turbines is constantly being challenged in seismically active areas. This study investigated the effect of ground motion directionality on the seismic dynamic responses of monopile offshore wind turbines (MOWTs). A set of horizontal pairs of ground motions were applied to a 5 MW MOWT model. Two groups of time-domain simulations, including (1) the wind turbine parked in a calm sea subjected to earthquakes, and (2) the wind turbine during normal operation at the rated wind speed subjected to earthquakes, were performed to investigate the seismic responses of the MOWT. The results showed that the structural responses of the MOWT were significantly affected by the angle of ground motion incidence. The environmental wind and wave loads contributed to a basic drift to the response of the structure, and can mitigate or increase the effect of ground motion directionality. Dynamic analyses using bi-directional horizontal ground motion exciting the wind turbine in the fore-aft (FA) and side-to-side (SS) directions resulted in a significant underestimation of the structural responses. The aerodynamic damping amplified the effect of ground motion directionality, and increased the underestimation of the structural responses using the FA and SS bi-directional excitation.