In-depth characterization of the genetic diversity and population structure of wild relatives is of paramount importance for genetic improvement and biodiversity conservation, and is particularly crucial when the wild relatives of crops are endangered. In this study, we sampled the Alpine plum (Briançon apricot) Prunus brigantina Vill. across its natural distribution in the French Alps, where its populations are severely fragmented and its population size strongly impacted by humans. We analysed 71 wild P. brigantina samples with 24 nuclear simple sequence repeat (microsatellite) markers and studied their genetic diversity and population structure, with the aim to inform in situ conservation measures and build a core collection for long-term ex situ conservation. We also examined the genetic relationships of P. brigantina with other species in the Prunophora subgenus, encompassing the Prunus (Eurasian plums), Prunocerasus (North American plums) and Armeniaca (apricots) sections, to check its current taxonomy. We detected a moderate genetic diversity in P. brigantina and a Bayesian model–based clustering approach revealed the existence of three genetically differentiated clusters, endemic to three geographical regions in the Alps, which will be important for in situ conservation measures. Based on genetic diversity and population structure analyses, a subset of 36 accessions were selected for ex situ conservation in a core collection that encompasses the whole detected P. brigantina allelic diversity. Using a dataset of cultivated apricots and wild cherry plums (P. cerasifera) genotyped with the same markers, we detected gene flow neither with European P. armeniaca cultivars nor with diploid plums. Similar to previous studies, dendrograms and networks placed P. brigantina closer to the Armeniaca section than to the Prunus section. Our results thus confirm the classification of P. brigantina within the Armeniaca section; it also illustrates the importance of the sampling size and design in phylogenetic studies.