Between 2015 and 2018, we provided training for 1806 municipal police officers in Tijuana, Mexico, in an effort to improve their knowledge and behaviors related to HIV and injection drug use. Correct knowledge of syringe possession laws improved from 56% before training to 94% after training and was sustained at 24 months (75%). Knowledge improvement was associated with decreases in arrests for syringe possession over time (adjusted odds ratio [AOR] = 0.87; 95% confidence interval [CI] = 0.85, 0.90). Officers with correct knowledge had significantly lower odds of reporting arrests (AOR = 0.63; 95% CI = 0.44, 0.89). Training was associated with sustained improvements in knowledge and practices that advance public health. (Am J Public Health. 2022;112(6):860-864. https://doi.org/10.2105/AJPH.2021.306702).