The need for effective HIV prevention programs, especially in Sub-Saharan Africa, remains urgent. We investigate the effect of a financial lottery program in Lesotho with relatively low expected payments but a chance to win a high prize conditional on negative test results for sexually transmitted infections. The intervention resulted in a 21.4% reduction in HIV incidence over two years. Lottery incentives appear to be particularly effective in targeting individuals with ex ante risky sexual behavior, consistent with the hypothesis that lotteries are more valued by individuals willing to take risks.