Despite the general expectation that ethical leadership fosters employees’ ethical behaviors, surprisingly little empirical effort has been made to verify this expected effect of ethical leadership. To address this research gap, we examine the role of ethical leadership in relation to a direct ethical outcome of employees: moral voice. Focusing on how and when ethical leadership motivates employees to speak up about ethical issues, we propose that moral efficacy serves as a psychological mechanism underlying the relationship, and that leader–follower value congruence serves as a boundary condition for the effect of ethical leadership on moral efficacy. We tested the proposed relationships with matched reports from 154 Korean white-collar employees and their immediate supervisors, collected at two different points in time. The results revealed that ethical leadership was positively related to moral voice, and moral efficacy mediated the relationship. Importantly, as the relationship between ethical leadership and moral efficacy depended on leader–follower value congruence, the mediated relationship was effective only under high leader–follower value congruence. Theoretical and practical implications are discussed.