Horseshoe crabs are an archetypal chelicerate group with a fossil record extending back to Early Ordovician time. Although extensively studied, the group generally has a low diversity across the Phanerozoic Eonothem. Here, we expand the known diversity of true horseshoe crabs (Xiphosurida) by the description of a new taxon from the Middle Triassic Strelovec Formation of the Slovenian Alps. The mostly complete fossil is preserved as an external mould and assigned to the family Limulidae Zittel, 1881 as Sloveniolimulus rudkini, n. gen., n. sp. The use of landmark and semilandmark geometric morphometrics is explored to corroborate the systematic palaeontology and suggests that the new genus and species are valid. We also provide the first quantitative evidence for the extensive diversity of Triassic horseshoe crabs. We suggest that Triassic horseshoe crabs likely filled many ecological niches left vacant after the end-Permian extinction.