Arenaria bernensis (Caryophyllaceae), traditionally placed within the Arenaria ciliata aggregate and discovered only in 1955, is a poorly studied endemic taxon of western Prealps in Switzerland. In this study, we aimed to deliver the first detailed appraisal on morphology, distribution and ecology of A. bernensis accompanied by first molecular insights into the A. ciliata aggregate using chloroplast (cp) DNA markers. Altogether 18 populations attributed morphologically to A. bernensis were found during our field surveys. The distribution area of A. bernensis forms an irregular line of ca. 50 km from the south-west (Dent de Lys, FR) to the north-east (Stockhorn, BE). The taxon counts globally ca. 4,000 individuals and the majority of populations are small (<100 individuals). Furthermore, our study demonstrates clear ecological and phytosociological differentiation between A. bernensis and Arenaria multicaulis, another member of the A. ciliata complex growing in the same geographic area. Arenaria bernensis grows exclusively on shady, cool and steep slopes with northern exposition in the alpine zone above 1,900–2,000 m a.s.l. Results of our chloroplast DNA analyses suggest that the A. ciliata aggregate is a monophyletic group (including A. ciliata s.str., A. multicaulis, A. gothica and A. bernensis). Additionally, A. bernensis is genetically closer to A. multicaulis than to A. ciliata s.str. However, our data also suggest that A. bernensis has a complex evolutionary history. Future investigation of nuclear genomic DNA is needed to clarify the phylogenetic status of A. bernensis. The present paper also delivers a first scientific drawing of A. bernensis and a detailed English description of its morphology. The here documented narrow endemic character of A. bernensis emphasizes the role of the western Prealps as a peripheral refugia during Quaternary glaciations.