Tsunamis in the Mediterranean Sea have been increasingly investigated in the last decades due to past destructive events. We present a novel approach for evaluating tsunami-induced coastal inundation, based on a generalization of Green’s law and a chain of intermediate and small-scale numerical simulations. At the intermediate level, simulations with a linear solver are made to identify the coastal distribution of a novel parameter, namely α, condensing all nearshore wave transformations other than shoaling. α represents a proxy for coastal susceptibility to the tsunami impact. Small-scale modelling of coastal flooding performed in the southern Italy on a freely available DTM at the Esaro river estuary (Calabria) yields inundation levels that compare well with those obtained via intermediate-scale modelling, with a saving in computational time of about 41%. This demonstrates the helpfulness of α to “scale” the offshore wave input and reduce the computational effort to evaluate flooding at regional scale.