Plants glabrous or glabrescent. Stems erect, much-branched, 0.1-0.8 m; proximal branches ascending. Leaves: petiole shorter than blade; blade narrowly linear-lanceolate to linear, 2-8 × 0.2-1.2(-1.7) cm, base narrowly cuneate to narrowly decurrent, margins erose, crispate, or irregularly undulate, apex acute to subobtuse. Inflorescences terminal spikes, erect, usually stiff. Bracts completely enfolding flower; of pistillate flowers with prominent excurrent midrib, venation distinct, broadly triangular to deltate, 5+ mm, longer than tepals, margins erose, crenate, or denticulate, apex acute or acuminate; of staminate flowers shorter than tepals, apex acute. Pistillate flowers: outer tepals rudimentary, less that 1.2 mm; inner tepals with venation distinct, 3-4(-5) mm, apex acute, with terminal mucro; style branches spreading; stigmas 3. Staminate flowers: tepals 5, equal or subequal, 2-3 mm, margins erose to denticulate, outer tepals with apex subobtuse to acute-acuminate; inner tepals with apex distinctly acuminate or mucronulate; stamens 5. Utricles light brown, elliptic to obovate-elliptic, 2(-2.5) mm, shorter than outer tepals, slightly rugose to smooth. Seeds dark reddish brown to brown, 1-1.3 mm diam., shiny.

Flowering summer-fall. Sandy areas, sand dunes, riverbanks, disturbed habitats; 1000-2000 m; Ariz., N.Mex., Tex., Utah; n Mexico (Chihuahua).

Amaranthus acanthochiton is critically imperiled in Utah and imperiled in Arizona; its Global Heritage Status Rank is G5 as defined by The Nature Conservancy. Seeds and young leaves of A. acanthochiton were used by Native Americans (Hopi) as food: the seeds cooked as a kind of porridge, and the leaves cooked and eaten as greens or with meat (D. E. Moerman 1998).