Soil water availability is the most important factor limiting crop yield worldwide. Understanding crop and weed transpiration in response to water supply may provide valuable insight into the mechanisms of crop yield loss in water-limited environments. A greenhouse experiment was conducted to quantify corn and velvetleaf transpiration in response to drying soil. Five plants of each species were well watered by adding back the equivalent water loss each day to reach field capacity, and five plants were subjected to drought stress (dry-down) by not replacing lost water. Normalized daily transpiration of dry-down plants was regressed on soil water content expressed as the fraction of transpirable soil water (FTSW). The critical soil water content below which plants begin to close their stomates occurred at FTSWcr = 0.36 ± 0.015 for corn and 0.41 ± 0.018 for velvetleaf. Total water transpired did not differ among species. Velvetleaf also responded to drought by senescing its oldest leaves, whereas corn mainly maintained its leaf area but with rolled leaves during peak drought stress. During a short-term drought, corn is expected to perform better than velvetleaf because it maintains full transpiration to a lower FTSW and does not senesce its leaves. Under severe long-term drought, the species that closes its stomates at greater FTSWcr will conserve water and increase its chances of survival. Moreover, senescing all but the youngest leaves may ensure at least some seed production. Research is needed to evaluate the effects of soil water supply on corn–velvetleaf interference in the field.