The development of rainfastness adjuvants is of great significance in reducing the loss of applied pesticides via rainfall and enhancing their efficacy. Most pesticides lose their efficiency after been exposed to 5 cm of rain. The use of rainfastness adjuvants reduces the number of times a pesticide must be applied to the field and, as a result, the overall quantity of pesticides used by a farmer. This not only reduces the costs incurred by farmers but also provides them with tools to enhance the sustainability of their operations. Also, enhancing the rainfastness of pesticides reduces the amount of pesticide runoff a farm operation generates. Two different mechanisms can provide rainfastness: fast uptake by the plant and strong adhesion to the plant surface. We studied a newly developed rainfastness adjuvant (Dow) that provides a significant improvement in rainfastness for multiple synthetic pesticides formulations. The newly developed adjuvant was added to the synthetic pesticides at a 7% concentration, and then the formulations were exposed to simulated rain at a flow rate of 6 L/h for different exposure times. An increase in active retention of up to 82% could be obtained with the newly developed adjuvant when compared to a control formulation. In addition, an increase in performance of up to 64% could be obtained with the adjuvant when compared to a commercial benchmark rainfastness adjuvant.