The rainfed lowland (RFL) rice ecosystem is prone to drought stress and high weed infestation, including weedy rice (WR), which often negatively affects the yield. This study examined the competitive ability of rice and WR under mixed culture and drought stress. Two rice genotypes (drought-susceptible IR64 and drought-tolerant INL73) and a WR were grown in root boxes with soil under mono (rice or WR only) or mixed (rice/WR) cultures and subjected to either continuously waterlogged (CWL) or progressive drought (PDR) conditions during the early vegetative stage. Under PDR, SDW in all planting cultures was reduced relative to their CWL counterparts. The WR in mixed culture had higher SDW than its monoculture counterpart, regardless of water treatments. SDW in mixed cultures under PDR was not affected in IR64, reduced in INL73, and increased in WR relative to their monocultures. Under PDR, water use (WU) was higher in INL73/ WR than in IR64/WR mixed culture. Between two mixed cultures, WR mixed with INL73 had higher SDW than when mixed with IR64. The differences in stomatal conductance between INL73 and WR under mixed culture were attributed to the greater increase in total lateral root length of the latter than the former genotype under PDR. Overall, the results showed that WR had greater competitive ability than rice under mixed cultures, especially with a drought-tolerant rice variety. This calls for careful planning of cultivar adoption in rainfed systems where WR is a problem.