Plants glabrous or sparsely pubescent in distal parts. Stems erect, green, branched, 0.3-1 m. Leaves: petiole of proximal leaves equaling or longer than blade, becoming shorter distally; blade rhombic-ovate or ovate to elliptic, 3-12 × 2-8 cm, base broadly cuneate, margins entire, apex slightly acuminate to obtuse and faintly emarginate, mucronate. Inflorescences terminal panicles and axillary spikes; panicles erect or often drooping, green, dense, branched, leafless at least distally. Bracts lanceolate, shorter than 2 mm, shorter than tepals, apex spinescent. Pistillate flowers: tepals 5, oblong-spatulate to oblong, not clawed, 1.5-2 mm, apex acute, often very shortly mucronate; style branches strongly spreading, shorter than body of fruit; stigmas 3. Staminate flowers usually clustered at tips of inflorescence branches, sometimes gathered in proximal glomerules (as in A. spinosus); tepals 5, equal or subequal; stamens 5. Utricles ovoid or subglobose, 1.5-2 mm, slightly shorter than tepals, smooth to irregularly wrinkled, dehiscence regularly circumscissile. Seeds dark reddish brown to black, subglobose or lenticular, 0.8-1 mm diam., shiny, smooth. 2n = 64.

Flowering summer-fall in tropics, various seasons in subtropics. Waste places, disturbed habitats; 0-100 m; introduced; Fla.; West Indies; South America; introduced and locally naturalized Europe, Asia, Africa.

Amaranthus dubius, a morphologically deviant allopolyploid, is very close genetically to both A. spinosus (sect. Centrusa) and members of sect. Amaranthus. This species most probably originated as a result of ancient hybridization between A. spinosus and either A. hybridus or A. quitensis (W. F. Grant 1959; T. N. Khoshoo and M. Pal 1972; M. Pal and T. N. Khoshoo 1965; J. D. Sauer 1967b; V. Srivastava et al. 1977). Amaranthus nothosect. Dubia Mosyakin & K. R. Robertson (A. sect. Amaranthus × A. sect. Centrusa), was proposed to accommodate A. dubius (S. L. Mosyakin and K. R. Robertson 1996).