Newly discovered species are often threatened with extinction but in many cases have received limited conservation effort. To guide future conservation, it is important to determine the extinction risk of newly described species. Here, we test how time since formal description of a species is linked to its threat status to obtain a better insight into the possible threat status of newly described species and as yet undescribed species. We compiled IUCN Red List data for 53,808 species from five vertebrate groups described since 1758. Extinction risk for more recently described species has increased significantly over time; the proportion of threatened species among newly described species has increased from 11.9% for species described between 1758 and 1767 to 30.0% for those described between 2011 and 2020. Based on projections from our analysis, this could further increase to 47.1% by 2050. The pattern is consistent across vertebrate taxonomic groups and biomes. Current species extinction rates estimated from data of all known species are therefore highly likely to be underestimated. Intensive fieldwork to boost discovery of new species and immediate conservation action for newly described species, especially in tropical areas, is urgently required.