Motion responses is a primary factor affecting the wave attenuation of floating breakwaters. This study aims at investigating the motion responses of a floating wave barrier (or known as H-Float) subjected to regular waves and a slack mooring using physical modelling. The motion reactions of the H-Float, which are empirically quantified by the response amplitude operators, RAOs for heave, surge and pitch, are related with the relative width and draft of the floating barrier as well as the wave steepness. The motion responses, particularly the surge and pitch, have been proven to significantly affect the wave attenuation of the breakwater. When subjected to slack mooring, the H-Float attains an optimum wave attenuation performance at a draft-to-water depth ratio of 0.13 regardless of the wave conditions. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021.