The present study reports on population dynamics and growth performance relative to a lesser known exotic invasive species (Brachidontes pharaonis) inhabiting the southern Mediterranean. The study was carried out in western Sicily, where B. pharaonis is present on both the submerged and emerged surfaces of a hyperhaline saltpan. Individuals were scraped, counted and measured for shell length, total weight, somatic, gonadic and shell biomass. Brachidontes pharaonis intensively colonized all hard substrates of the saltpan with annual average densities of 375 ± 293 ind. 400 cm−2 with density peaks in autumn as a function of habitat. The occurrence of juveniles was different for mediolittoral and infralittoral populations as was mean size, spawning periods and annual organic matter biomass. Organic matter allocated to the shells represented 56.4% (of the total), gonad allocation was 7.3%, while somatic allocation averaged 36.3% for both populations. Brachidontes pharaonis reached high individual density, rare among temperate members of the genus Mytilus in Mediterranean and European waters, but common in tropical dense mussel beds. Considering its invasive potential and the recent warming trend of the Mediterranean, in the future B. pharaonis may actively invade more habitats, threatening indigenous bivalve species which may be unable to compete with B. pharaonis in terms of reproductive effort and density.