Perennials monocarpic, 40-150 cm; taprooted. Stems usually 1, erect, less commonly several, ascending, simple to sparingly short-branched in distal ¹/2, less commonly openly branched, villous with jointed trichomes; branches on distal stems 0-many, short, ascending. Leaves: blades linear to oblong or elliptic, 10-30+ × 1-10 cm, margins flat to undulate, subentire to coarsely dentate or shallowly to deeply pinnatifid, lobes lance-oblong to broadly triangular, spinulose to spiny-dentate or shallowly lobed, main spines 3-12 mm, abaxial faces green and subglabrous to gray- or white-tomentose with felted arachnoid trichomes, ± villous to tomentose along major veins with septate trichomes, rarely glabrous or glabrate, adaxial ± green, glabrous or villous with septate trichomes; basal often present at flowering, spiny winged-petiolate or sessile; principal cauline well distributed, proximally winged-petiolate, distally sessile, gradually reduced, bases sometimes short-decurrent (0-2 cm); distal ± reduced, often narrower than the proximal, sometimes with non-pigmented bases. Heads several-many, erect, usually in racemiform or spiciform arrays, usually closely subtended by clustered ± leafy bracts. Peduncles 0-15+ cm. Involucres (green), broadly ovoid, 1.5-2.5 × 1.5-2.5 cm, loosely villous with septate trichomes, sparingly if at all arachnoid. Phyllaries in 4-8 series, subequal, ± lanceolate, bases appressed, apices ascending, linear to broadly expanded, erose to lacerate or spiny-fringed, spines straight, slender, 2-3 mm, abaxial faces with or without narrow glutinous ridge; apices of inner flexuous, sometimes expanded and erose. Corollas white, ochroleucous, 19-23 mm, tubes 6.5-8.5 mm, throats 7.5-11 mm, lobes 4-5 mm; style tips 4-5.5 mm, conspicuously exserted. Cypselae brown, 5.5-6.5 mm, apical collars not differentiated; pappi 17-20 mm. 2n = 34.

Flowering summer (Jun-Aug). Moist soil, roadsides, meadows, forest edges and openings; of conservation concern; 1500-2400 m; Mont.

Cirsium longistylum is endemic to the Big Belt, Castle, Elkhorn, and Little Belt ranges of west-central Montana. It is highly variable, and several authors have suggested that it has introgressed with one or more other species (R. J. Moore and C. Frankton 1963; J. M. Poole and B. L. Heidel 1993; S. J. Brunsfeld and C. T. Baldwin, unpubl.). It is closely related to C. hookerianum, and the two probably share a common ancestry or a history of hybrid interactions dating back to the Pleistocene. Cirsium longistylum is perhaps also affected by modern or historic introgression involving C. scariosum var. scariosum.