A form of Theba pisana with the shell keeled along the periphery and an open umbilicus has been known from sand dunes in south-western Portugal since 1989 when shells were reported from near Monte Clérigo, Algarve. It was initially identified as T. p. arietina (Rossmässler, 1846), a rare taxon otherwise reported only from Cádiz province in Spain, and this name has been adopted in all subsequent literature on the Portuguese populations. Comparisons of topotypical Spanish T. p. arietina with samples from Monte Clérigo show that these are indeed similar in having ± large keeled shells that are pale coloured and usually unbanded, but differ in several characters and do not represent the same taxon. However, there are much more extensive populations on sand dunes in south-western Portugal, very different in appearance, with smaller shells, a blunt keel, larger umbilicus and usually banded. These are likely to have originated independently from the Spanish T. p. arietina, and are therefore named as a new subspecies, T. p. almogravensis. Three extensive areas of coastal dune vegetation (each 2.5-4.8 km2) up to ca 82 km apart support large populations of it with almost constant shell characters. Descriptions of shells and genital anatomy are provided for both T. p. arietina and T. p. almogravensis. Shell samples from six smaller coastal areas (0.1-0.3 km2) of sand dune or transitional habitat in SW. Portugal, including Monte Clérigo resembling T. p .arietina, differ widely between populations and appear to show nearly stable combinations of characters of T. p. almogravensis and T. p. pisana. They are probably of hybrid origin, since similar forms arise among the much more variable “hybrid swarms” on some disturbed margins of habitats of typical T. p. almogravensis, where it meets T. p. pisana, and where mixed pairs were seen mating. Threats to T. p. almogravensis may therefore arise from introgression with T. p. pisana, but to ascertain this, further research is needed on changes in local populations and their habitats.