Greenhouse experiments were conducted to evaluate the influence of three organosilicone surfactants (Silwet L-77®, Silwet® 408, and Sylgard® 309) and two blends of organosilicone surfactants with conventional adjuvants (Dyne-Amic® and Kinetic®) on the rainfastness of primisulfuron in velvetleaf. Four conventional adjuvants, Rigo Oil Concentrate, Agri-Dex®, methylated soybean oil, and X-77® were selected for comparison. Primisulfuron at 40 g ai/ha was applied alone or with the organosilicones, blends, or X-77 at 0.25% (v/v); the other adjuvants were tested at 1% (v/v). Simulated rainfall (1.25 cm/0.5 h) was applied at 0.25, 0.5, 1, or 2 h after treatment. Control ratings were made at 5-d intervals and shoot fresh weights were recorded 20 DAT. The organosilicone surfactants significantly increased the rainfastness of primisulfuron, even when simulated rainfall was applied at 0.25 or 0.5 h after treatment. Rigo Oil Concentrate and Kinetic also increased rainfastness when rainfall was applied 1 h after treatment. Differences among adjuvants were not as apparent when rainfall was applied 2 h after treatment. Regardless of the timing of simulated rainfall after treatment, there were significant differences between treatments and nontreated check; however, control was not acceptable in several instances.