The distribution of the allochthonous crab Percnon gibbesi and its relationships with other benthic invertebrate species was assessed inside the marine reserve of Cabo de Palos—Islas Hormigas (Mediterranean sea, Spain) and neighbouring non-protected sites. Although a significant spatial variability was detected at finer spatial scale, there was no influence of protection measures or insularity on the abundance of P. gibbesi. The presence of small holes, encrusting algae and low slope favour the colonization success of this crustacean, and the spatial distribution of these habitat features could explain the observed pattern. The abundance of P. gibbesi was similar to that of native crab species; however, a non-significant negative relationship between the abundance of P. gibbesi and native crabs (Pachygrapsus marmoratus and Eriphia verrucosa), urchins (Arbacia lixula and Paracentrotus lividus) and a snail (Phorcus turbinatus) was observed. This work highlights the importance of monitoring alien crab population densities taking structural habitat and other potentially influential factors into account and the likely effect of this alien species on the native ones.