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.
Phenology: Flowering summer–fall in tropics, various seasons in subtropics.
Habitat: Waste places, disturbed habitats
Elevation: 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).