Rhizomes very short, densely tufted. Stems 5-20 cm × 0.4-0.7 mm, trigonous, rough, stiff, greyish-green, leafy at base. Leaves mostly basal, narrowly linear, 3-12 cm × 0.5-1.5(-2) mm, shorter than stems, gradually narrowed to a shortly trigonous, ± obtuse apex, channelled, greyish-green, with margins scabrid near apex; sheaths 1-3 mm long, dull orange-brown to mid brown; ligule c. 0.4 mm long, truncate at apex. Lowest involucral bract comprising a sheath with a short (c. 3-5 mm) greenish to brown lamina. Inflorescence a cylindric to ovoid-cylindric, 1-2.5 mm long terminal cluster of 3-10 spikes. Spikes 0.4-0.8 mm long, usually crowded, rarely the lowest somewhat distant, mid brown to mid reddish-brown, with male florets above and female below in each spike. Glumes 4-5 × 1.3-1.6 mm, those of male and female florets similar, elliptic-ovate, with reddish-brown keel and greenish membranous sides with scarious margins; apex obtuse to acuminate-aristate. Flowers unisexual; perianth bristles 0; stamens 3; anthers c. 2 mm long; stigmas 3; prophyll (utricle) open at least towards the top, leaving nut exposed, frequently also open down the side, often more so as nut matures. Nuts 2-3 × c. 1.5 mm, oblong, trigonous, pale dull brown.
Kobresia is closely related to Carex, the former being distinguished by the fact that the utricle does not fully enclose the nut, being open at least at its apex and often down its side. In addition each spike has a male floret at the top and a female one below.
Kobresia simpliciuscula is very locally distributed, being known only from Teesdale, Mid and East Perthshire (v.cc. 88 and 89) and Argyll (v.c. 98), but some populations contain tens of thousands of plants (Porter & Foley 2002).
Kobresia simpliciuscula is characteristic of highly calcareous, somewhat hummocky open flushes within the Briza media-Primula farinosa subcommunity of the Carex dioica-Pinguicula vulgaris mire (M10b), and there distinguished as a variant, M10biii, best seen in Teesdale, in which Racomitrium lanuginosum and Carex capillaris are often prominent with the dominant Kobresia. In Scottish localities Carex dioica, C. hostiana, C. panicea, C. pulicaris and C. viridula subspp. brachyrhyncha and oedocarpa, Eleocharis quinqueflora, Juncus articulatus and the bryophytes Aneura pinguis, Ctenidium molluscum and Palustriella commutata amongst others, are common and Equisetum hyemale and Schoenus ferrugineus locally frequent.
Months of the fruiting period: 7-8.