Arenaria microcalyx M. Koc & Hamzaoglu (Caryophyllaceae) is described as a new species of subgen. Arenaria L. in Turkey. The specimens were collected from Kas (Antalya). The new species is related to Arenaria leptoclados (Rchb.) Guss. and is an endemic species that has a distribution in the Mediterranean region. The differences between the species are discussed. Description, distribution, illustration, and conservation status of the new species are given. Seeds structures were examined by scanning electron microscopy.