Background: Thai farmers are directly exposed to pesticides, which may result in adverse effects including cognitive impairment. Objective: The aim of this study was to examine the association between occupational pesticide exposure and cognitive decline among adult farmers in northern Thailand. Material and Methods: This cross-sectional study included 303 pesticide-using farmers over the age of 50 from Doi Tao District in Chiang Mai Province. Pesticide exposure score was calculated using an algorithm that considered personal protective equipment (PPE) scores and exposure intensity scores, as well as lifetime application days. The scores were classified as high or low exposure based on their median. The Thai version of the Montreal Cognitive Assessment (MoCA) test was used to assess cognitive function. Results: The mean age of adult farmers was 58.74 years. The prevalence of cognitive impairment was 93.7%, with an average score of 19.6. Spearman’s rank correlation coefficient showed that the MoCA score was adversely correlated with lifetime application days (r_s = -0.145), PPE score (r_s = -0.163), exposure intensity score (r_s = -0.184), and pesticide exposure score (r_s = -0.225). Linear regression revealed that high exposed farmers had significantly lower MoCA scores than low exposed farmers, as measured by PPE score (B = -0.75; 95% CI: -1.46, -0.05), exposure intensity score (B = -0.97; 95% CI: -1.66, -0.27), and pesticide exposure score (B = -0.77; 95% CI: -1.47, -0.06), after controlling for sex, age, education, income sufficiency, and body mass index. Conclusions: Thai farmers are at risk of cognitive impairment linked to occupational pesticide exposure, depending on their PPE use and exposure intensity. There is still a critical need for action to reduce the risk of negative health effects from pesticide exposure among Thai farmers.