Cap up to 1.0 cm in diameter, convex to umbonate, margin usually smooth, but can be crenulate, becoming upturned with age, tacky when moist, with slight spotting, not hygrophanous, uniformly colored or bicolored, at center clay buff or pale yellowish brown or cinnamon, at margin paler, universal veil absent, partial veil present. Lamellae clay color, emarginate to adnate, maximum depth not recorded, number of lamellae {L}18–28; droplets absent, white fimbriate edge indistinct. Stem 1.0–2.0 × 0.2–0.3 cm, ochre-brown, paler in apex, cylindrical, stem Q 6.0–8.5, fibrillose, pruinose at apex. Context firm, stem interior stuffed, stem flesh not discoloring from base. Smell weakly raphanoid or insignificant. Taste not recorded. Spore deposit not recorded.

Spores amygdaloid or limoniform, very strongly papillate, on ave. 14–16 × 8.5–9.5 µm, ave. Q = 1.6–1.8, pale brown to yellow brown, weakly guttulate, almost smooth to weakly ornamented (O1O2), perispore not loosening (P0), distinctly to rather strongly dextrinoid (D2D3). Basidia 23–44 × 6–10 µm, ave. Q = 3.1–4.3, characteristically 2-spored. Cheilocystidia lageniform or ventricose, occasionally clavate-ventricose, characteristically geniculate and septate, on ave. 40–70 × 5.5–6.5 (apex) × 4.5–5.5 (middle) × 8–10.5 (base) µm, ratios A/M = 1.02–1.44, A/B = 0.53–0.75, B/M = 1.48–2.03. Epicutis an ixocutis, up to 130 µm thick (measured from exsiccata), maximum hyphae width 6 µm, ixocutis hyphae sometimes encrusted, shape of trama elements beneath subcutis isodiametric. Caulocystidia similar to cheilocystidia, up to 120 µm long.

N-Greenland: Zackenberg, Sydkæret, 74.50°N, 20.75°W, 19 Aug 1999, T. Borgen (TB99.376, C-F-104295), 30 m, with Salix arctica. E-Greenland: Jameson Land, Nerlerit Inaat/Constable Pynt, west side of Nathorst Fjeld, 70.76°N, 22.64°W, 5 Aug 2017, H. Knudsen (HK17.101, C-F-104997), 40 m, with Salix sp. in tundra.

Recently described from the southern Carpathians in Romania from a single collection. The two Greenland collections are the first recorded since the type has been published. Both are from the High Arctic zone in Greenland at 70° and 74°N, and it is thus in the northernmost group among the 28 Greenland species. The records are the first from North America and the first outside Europe.

Only two records, both most likely with Salix arctica and both on calcareous soil. The only previous collection, the type, grew with S. retusa L. at 2270 m a.s.l. in the Carpathians (Eberhardt et al. 2015a). Shiryaev et al. (2018) reported from Zmitrovich and Ezhov (2015) the presence of Hebeloma gigaspermum Gröger & Zschiesch. from Franz Josefs Land (Russia). According to Beker et al. (2016), H. gigaspermum is most likely a synonym of H. nauseosum Sacc. Since neither H. nauseosum nor any other species of H. sect. Sacchariolentia occurs in arctic areas, it may be that the large spores and the occurrence in the High Arctic zone rather point to H. grandisporum.

Hebeloma grandisporum is one of the most easily recognized among the arctic-alpine Hebeloma species by the small basidiomes, the large spores, the relatively few, distant lamellae and large, 2-spored basidia. To date, it is the only Hebeloma, of which we are aware, that has consistently 2-spored basidia.