Vegetable amaranth (Amaranthus spp.) is a promising C₄ crop for semi-arid regions due to its high nutritive value and an ability to adapt to drought stress. A pot experiment in a temperature-controlled greenhouse was conducted to investigate the effects of drought stress on biomass production, partitioning, and water use efficiency (WUE) of four genotypes of vegetable amaranth, viz. 'Hin Choi' (A. tricolor), 'Co.2' (A. tricolor), 'WS80-192' (A. blitum), and 'RRC 1027' (A. cruentus). Drought stress significantly decreased plant total dry mass, but the proportion of changes differed among root, stem, and leaf. Under drought, root dry mass ratio was increased in Co.2, WS80-192, and RRC 1027, whereas leaf dry mass ratio was decreased in Hin Choi, Co.2, and RRC 1027. Leaf area per root dry mass was decreased by drought stress in all genotypes examined. These results indicate that drought induced a more conservative balance between water-losing and water-obtaining organs. Specific leaf area (SLA) was decreased by drought stress and differed between genotypes. WUE of the four genotypes of vegetable amaranth was unaffected by drought stress, and ranged from 2.9 to 3.8 g DM kg^-1 H₂O, being the highest in RRC 1027 and the lowest in WS80-192. A negative relationship between SLA and WUE was observed over the four genotypes of vegetable amaranth under well-watered conditions.