The present paper concerns the extreme wave loads acting on an offshore structure; specifically the wave-in-deck loading component that arises when the height of an incident wave crest exceeds the elevation of the topside structure. In this case wave inundation occurs, the resulting loads on the topside structure represent a significant part of the total wave load. A new model for the effective prediction of this important loading component is presented. This is based upon the conservation of momentum, is formulated in a Lagrangian frame of reference, can incorporate any incident wave form, and takes due account of the porosity (or openness) of the topside structure. Comparisons between the model predictions and wide-ranging laboratory observations are shown to be in good agreement; the latter based upon deterministic focused wave events that are known to be representative of the largest waves arising in realistic sea-states. In addition, comparisons are also made with independent cfd calculations. Taken together, the proposed model is shown to accommodate changes in the spectral shape, the spectral peak period, the incident crest elevation (and hence the level of inundation), the directional spread of the incident waves, and the porosity of the topside structure. Importantly, this agreement applies to both non-breaking and breaking waves, involves no empirical calibration, and can be achieved with limited computational resources. As such, the model is ideally suited to design/re-assessment calculations in which the reliability of any offshore structure must be based upon a rigorous assessment of the long-term distribution of the total wave loads, including any wave-in-deck loading component.