The Mesovoid Shallow Substratum (MSS) is a largely unknown subterranean habitat formed by the accumulation of rock fragments in steep slopes. In this study, we investigated the diversity, abundance and seasonal dynamics of terrestrial isopods in MSS habitats across Portugal. We identified 12 species belonging to five families, with Porcellionidae and Armadillidiidae being the most represented. The new species, Eluma cristata, endemic to the MSS of Alcobertas, is described. The terrestrial isopod communities included both surface-dwelling and subterranean-adapted species, pointing out the connectivity between these habitats and highlighting the adaptability of terrestrial isopods to diverse environments. Widespread species occurring in multiple locations, and species confined to single sites are present. Seasonality does not seem to affect the community composition. Our findings highlight the rich diversity of MSS habitats, thus their importance for conservation.