tThe herbicide S-metolachlor is used to control or suppress annual grasses, nutsedges and several broadleafweeds in sweetpotato. However, a decline in storage root quality is suspected when an excessive rainfalloccurs within 24 h after application. A sunlit, controlled environment experiment was conducted to inves-tigate sweetpotato response to five levels of S-metolachlor (0.00, 0.86, 1.72, 2.58 and 3.44 kg ha−1), andtwo levels of simulated rainfall (0 and 38 mm at 51 mm h−1) immediately after application. Sweetpotatoslips were transplanted into white polyvinyl chloride pots filled with sandy loam soil. S-metolachlor treat-ments were applied to slips and a simulated rainfall treatment delivered immediately after transplantingand herbicide treatment. All pots were transferred to sunlit growth chambers that were maintained at30/22◦C, day/night temperatures and ambient carbon dioxide concentration (400 L L−1) for 60 days. Anevapotranspiration-based irrigation system was used to supply water and nutrients. Plant biomass com-ponents and quality of storage roots were recorded 60 days after transplanting. There was no differencebetween rainfall treatments across S-metolachlor rates for vine lengths, leaf numbers and leaf area. Theseparameters, however, declined linearly and significantly with increase in S-metolachlor concentration.Total storage root weight declined linearly with increased S-metolachlor concentration; the decline wassteeper with simulated rainfall. Yield of marketable storage roots declined by 18 and 31% in the absenceof rainfall and 55 and 79% in the presence of rainfall with S-metolachlor at minimum (0.86 kg ha−1) andmaximum (1.43 kg ha−1) recommended label rates, respectively, used to control weeds. Yield reductionwas directly proportional to the rate of S-metolachlor applied in the absence or presence of rainfall; 77and 123 g fresh weight kg−1ha−1S-metolachlor for no-rainfall and rainfall treatment, respectively. Theseresults can be used to improve management decisions to optimize yield under field conditions as well asto mitigate risk of injury that could be associated with the use of S-metolachlor in sweetpotato productionsystems.