This study examines the impact of major customer concentration on corporate profitability among manufacturing firms listed in the Korean stock market from 2013 to 2024. The main findings are as follows. First, customer concentration has a negative effect on corporate profitability. Second, the negative relationship between customer concentration and profitability is mitigated as the ownership share of foreign investors increases. The results suggest that, in the Korean market, a higher dependence on a few major customers enhances the bargaining power of those customers, which in turn reduces the profitability of supplier firms. However, this adverse effect is alleviated when the proportion of foreign investors—who possess greater expertise—increases, as they effectively perform monitoring and advisory roles, thereby enhancing corporate stability.