Three trials (two in 2019 and one in 2020) were completed at the University of Guelph, Huron Research Station near Exeter, ON, to determine if the co-application of thifensulfuron with glyphosate accentuates soybean injury in glyphosate-resistant (GR) soybean. At 1, 2, 4, and 8 weeks after treatment (WAT), thifensulfuron (6 and 12 g a.i.·ha⁻¹ representing the 1× and 2× rate, respectively) applied POST with no adjuvants caused up to 5% soybean injury. The addition of a non-ionic surfactant + UAN to thifensulfuron increased soybean injury to up to 24%. There was no decrease in soybean density, dry biomass, height, and yield, except soybean dry biomass was reduced up to 22% with the addition of adjuvants to thifensulfuron at the 2× rate. Glyphosate (1800 and 3600 g·ha⁻¹ representing the 1× and 2× rate, respectively) applied POST caused no adverse effect on soybean injury parameters evaluated. The co-application of glyphosate + thifensulfuron at the 1× and 2× rates, without additional adjuvants, caused a synergistic increase in soybean injury at 1, 2, 4, and 8 WAT, and a synergistic decrease in dry biomass and height. All other interactions were additive. The co-application of glyphosate + thifensulfuron at the 1× and 2× rates, with additional adjuvants, produced a synergistic increase in injury at 1 (1× and 2× rate), 4 (1× rate), and 8 (1× rate) WAT in soybean. All other interactions were additive.