There is little information on the biologically effective rate of halosulfuron applied after emergence (postemergence, POST) for the control of broadleaf weeds in corn under Ontario environmental conditions. Six field trials were conducted over a 2-yr period (2014–2015) to determine the biologically effective rate of halosulfuron applied POST for the control of velvetleaf, pigweed species, common ragweed, common lambsquarters, and eastern black nightshade in corn. Based on regression analysis, the predicted halosulfuron rates required to cause 5%, 10%, and 20% corn injury were 53, 138, and >560 g a.i. ha^-1 at 1 week after application (WAA), 109, 276, and >560 g a.i. ha^-1 at 2 WAA, and 493, >560, and >560 g a.i. ha^-1 at 4 WAA, respectively. The predicted halosulfuron rates applied POST for 95% control of velvetleaf, pigweed species, common ragweed, common lambsquarters, and eastern black nightshade were 10–13, 35–143, 25–57, >560, and >560 g a.i. ha^-1, respectively. The predicted halosulfuron rates applied POST to reduce velvetleaf, pigweed species, common ragweed, and common lambsquarters density by 80% were 9, 9, 7, and >560 g a.i. ha^-1, respectively. The predicted halosulfuron rates applied POST to reduce velvetleaf, pigweed species, and common ragweed dry weight by 80% were 2, 3, and 2 g a.i. ha^-1, respectively. Based on these results, halosulfuron applied POST at the registered rate of 34–68 g a.i. ha^-1 has the potential to control velvetleaf, pigweed species, and common ragweed but does not adequately control common lambsquarters and eastern black nightshade in corn.