Maculinea nausithousis the most endangered Iberian butterfly, being included in the Bern and Habitat directives and in the Spanish Red Book for invertebrates. We map its potential distribution in the Iberian Peninsula, identifying potentially suitable areas where to focus future surveys. Using presence/absence data and a set of environmental variables, the distribution of M. nausithous was modelled using Generalized Additive Models. Model results were filtered using land cover data to identify the anthropogenic grasslands inhabited by the species. The model obtained indicates that some Iberian areas that currently do not support M. nausithous populations could be environmentally suitable for the species. The reasons for these absences are discussed. Indeed, field surveys identified possible sources of uncertainty at finer scale, highlighting that deficiencies in land use GIS data might constitute an important source of error, able to explain both commission and omission errors (i.e., over and underpredictions) of the model.