The influence of soil moisture on the competitive relationship between yellow nutsedge and cotton was determined in an additive competition experiment where main plots were irrigation regimes and subplots were nutsedge densities. Increasing nutsedge density caused a significant linear decrease in seed cotton yield. Regression analysis indicated that approximately 9 (1993), 12 (1994), or 37 (1995) kg ha−1 of seed cotton yield was lost for each additional initial nutsedge tuber per meter of crop row. Both postharvest stem biomass, an indicator of total aboveground biomass, and seed cotton yield increased with increasing soil moisture in all 3 yr of the study. Increasing yellow nutsedge density reduced midseason cotton height in both wet and dry soil moisture regimes but only slightly reduced postharvest stem biomass in the wet treatment in 1995. There was an interaction between soil moisture regime and yellow nutsedge density only with respect to midseason cotton leaf water potentials. Soil moisture availability did not interact with the effect of yellow nutsedge density with respect to end of the season stem biomass or seed cotton yield.