This study examines how antitrust law adoptions affect horizontal merger and acquisition outcomes. Using the staggered introduction of competition laws in 20 countries, we find antitrust regulation decreases acquirers’ 5-day cumulative abnormal returns surrounding horizontal merger announcements. A decrease in deal value, target book assets, and industry peers’ announcement returns are consistent with the market power hypothesis. Exploiting antitrust law adoptions addresses a downward bias to an estimated effect of antitrust enforcement. The potential bias from heterogeneous treatment effects does not nullify our results. Overall, antitrust policies seem to deter post-merger monopolistic gains, potentially improving customer welfare.