Plants glabrous. Stems erect or ascending, branched, 0.1-0.5 m. Leaves: petiole 1/2 as long as blade; blade oblong-lanceolate to lanceolate-linear, 1-3 × 0.2-1 cm, base cuneate to narrowly cuneate, margins entire, plane or sometimes undulate, apex rounded or obtuse-truncate, mucronate. Inflorescences mostly axillary, but at apex flowers also condensed in terminal spikes or spicate panicles, usually leafy proximally or nearly leafless distally. Bracts broadly ovate, 1 mm or shorter, 1/2 or less length of tepals, apex acute. Pistillate flowers: tepals 5, spreading at maturity, spatulate, clawed, subequal, 2 mm, margins fimbriate, apex rounded or shallowly emarginate; stigmas 3. Staminate flowers: tepals (3-)5; stamens 3. Utricles narrowly oblong, 1-1.4 mm, shorter than tepals, indehiscent. Seeds dark reddish brown to nearly black, lenticular or broadly lenticular, 0.6-0.8 mm diam., smooth.
Flowering summer-fall. Semideserts, naturally disturbed habitats; 1000-1200 m; Ariz., Tex.; n Mexico (Sinaloa, Sonora).
Amaranthus obcordatus has been reported only from southern Arizona (T. H. Kearney and R. H. Peebles 1960), trans-Pecos Texas in Brewster and Pecos counties, and adjacent regions of Mexico (C. F. Reed 1969b). It was also tentatively reported for New Mexico, but no localities were cited (W. C. Martin and C. R. Hutchins 1980). The proper taxonomic position of A. obcordatus and its relationships to other species remain problematic and are in need of further study.