Stems decumbent to erect, forming dense mats, hairy. Leaves closely imbricate, usually appressed to pair above, usually 4-ranked (decussate); blade narrowly triangular, dorsi-ventrally expanded marginally forming abaxial groove, 3-6 × 1-1.5 mm, margins thin, without curled hairs at leaf tips, surfaces with short, stiff pubescence. Flowers: sepals 2-3 mm, margins entire; petals connate to 2/3 their lengths, tips spreading to recurved, corolla white to yellowish, cylindric, 4-10 mm; stamens 2 mm. Capsules 3-5 mm. 2n = 26.

The Arctic plants of Cassiope tetragona all belong to subsp. tetragona, which is partially replaced by and partly sympatric with subsp. saximontana in northern Cordilleran areas of western Canada, Alaska, Montana, and Washington. The latter differs in pedicel length relative to leaf length and the length of the corolla.