Redroot pigweed (Amaranthus retroflexus L.) is among the most troublesome weeds in the Intermountain West affecting corn (Zea mays L.) production and contributing to significant yield losses, in addition to losses caused by water stress. Improvements in agricultural technology such as use of drought-tolerant (DT) corn hybrids has helped minimize the impact of water stress on corn yields. However, it is not known how the use of hybrids affects the interactions between weeds and corn. This work evaluated the competitive effects of A. retroflexus on DT and drought-susceptible (DS) corn hybrids exposed to optimal and reduced irrigation levels in a semi-controlled study. The semi-controlled environment was established in a rainout shelter with corn maintained at a density of 66,482 plants ha−1 and A. retroflexus varied at densities of 0, 33,241, and 66,482 plants ha−1 that were then provided either optimal or reduced irrigation (100% and 50%). We observed a 45% reduction in the shoot biomass of DS corn under reduced irrigation, while the shoot biomass of DT corn remained the same under both irrigation levels in Season 1. In Season 2, both hybrids experienced a decrease in shoot biomass under reduced irrigation. Amaranthus retroflexus exhibited an 80% increase in shoot biomass when growing with DS corn exposed to reduced irrigation, compared with its growth with DS corn exposed to optimal irrigation. Conversely, DT corn negatively impacted A. retroflexus shoot biomass under reduced irrigation, resulting in only a 9% difference between the reduced and optimally irrigated plots. These findings suggest that DT corn may mitigate water stress while also providing the additional benefit of improved competition against weeds, effectively suppressing their growth in water-stressed environments.