A new species of Cyphophthalmi belonging to the Laurasian family Sironidae is described and illustrated on the basis of external anatomy and genitalia. Siro crassus sp. n., known from leaf litter from two beech forest localities in Eastern Slovenia, is a relatively large (2.2–2.6 mm in length), long-legged epigean species, in which the patella of leg IV surpasses the end of the body. The new species resembles the European Siro rubens Latreille, 1804 and S. valleorum Chemini, 1990 in the projecting female rear end, but differs from these species, among other features, in having ornamented metatarsi on all four pairs of walking legs.