Abstract The plant Carduncellus matritensis (Cardueae, Compositae) was formerly known from only a single locality: Cerro Negro in Madrid Province, Spain. It is one of the six endemic species of the Spanish flora considered to be extinct. The causes of its presumed extinction were anthropogenic: the growth of the city of Madrid subsumed the locality into the city, such that Cerro Negro is now a railway station. During 2021–2022 we discovered three new populations c. 50 km south of the type locality, with 130–166 mature individuals. Habitat fragmentation and degradation are the most significant threats facing the three populations. We categorize the conservation status of the species as Critically Endangered according to the IUCN Red List criteria. To avoid the loss of this species, we recommend both in situ (creation of micro-reserves) and ex situ (seed storage) conservation measures.