This study examines the effect of product market competition on stock returns by treating the establishment of the US−Korea Free Trade Agreement (KORUS FTA) as an external shock. We use the degree of tariff reduction resulting from the KORUS FTA to construct a unique dataset for measuring product market competition. We sort firms into high-tariff-change (treatment group) and low-tariff-change groups (control group) and use the controlled difference-in-differences method to mitigate any possible endogeneity problems. We find clear empirical evidence that product market competition significantly negatively affects stock returns.