The rocky habitats of southern Portugal are ecosystems with extreme xericity conditions, associated with special abiotic strains. In these unstable ecological conditions, a considerable diversity of plant communities occurs. The objective of this study, carried out in the Algarve and Monchique, and the Mariánica Range biogeographical sectors, is to compare chasmo-chomophytic communities of the southwestern Iberian Peninsula, using a phytosociological approach (Braun–Blanquet methodology) and numerical analysis (hierarchical cluster analysis). From these results, two new communities were identified, Sanguisorbo rupicolae-Dianthetum crassipedis and Antirrhinetum onubensis, as a result of floristic and biogeographical differences from other associations already described within the alliances Rumici indurati-Dianthion lusitani and Calendulo lusitanicae-Antirrhinion linkiani, both included in the Phagnalo saxatilis-Rumicetea indurate class.