A new species of the Carex flava complex (Cyperaceae) is described from the Czech Republic. It is known only from the type locality and is assumed to be endemic to the Krkonoše Mts. Its systematic position along with karyological and ecological notes are presented here. The new entity proposed, Carex derelicta, is included in the subsection Serotinae of the section Ceratocystis. The distinctive features of this species are its combination of globose to shortly cylindrical female spikes, glumes of female spikes equalling or exceeding the perigynia; perigynia 2.0–2.5 mm long, not inflated, vivid green, beaks 0.4–0.7 mm long and achenes completely filling perigynia. The chromosome number n = 35 is the first reported for this taxon.