Exotic herbivores can exert profound impacts on terrestrial communities, but their ecological effects on marine habitats are not sufficiently quantified. The exotic crab Percnon gibbesi, which is rapidly spreading throughout the Mediterranean Sea, grazes almost exclusively on benthic macrophytes, providing an opportunity to study the potential impacts of herbivores in the marine realm. Here, we first quantified the abundance of P. gibbesi in Mallorca (Balearic Islands; Spain) in 2023 and reported average densities of 67 individuals 100 m-2, approximately 33 times greater than those recorded in 2003 on the islands. We then performed a feeding preference experiment using common native and invasive species of macroalgae (Caulerpa cylindracea, Halimeda incrassata, Haliptilon virgatum, Halopteris scoparia, Padina pavonica and Ulva compressa) from the Mediterranean Sea. The per capita grazing rates of P. gibbesi (3.83 ± 1.71 WW g crab -1 day-1), which can ingest almost 75% of their body weight daily, were higher than those recorded for most native herbivorous species in the Mediterranean. The estimated daily grazing rates for P. gibbesi average 23.98 ± 15.45 kg WW macroalgae ha-1 d-1, value that corresponds with 0.2 to 9.1% (average 5.58%) of the total macroalgae production in this area. Our experiment revealed clear preferences of P. gibbesi for three species of macroalgae, which were not explained by the nutritional content. Overall, our results generate great concern and coupled with the large extent of the invasion, indicate that the ecological impacts of this exotic herbivore on Mediterranean marine communities could be substantial and, until now, unreported.