SummaryDrawing on uncertainty management theory, we propose that employees' uncertainty is a distinct key mechanism explaining the relationship between ethical leadership and employees' organizational citizenship behavior (OCB). We contend that ethical leadership, by promoting a work environment governed by moral principles, reduces employees' sense of uncertainty and thereby fosters their OCB. However, we suggest that leaders' close monitoring, which ethical leaders may utilize to be informed about employees' adherence to their moral standards, nullifies these positive implications of ethical leadership. To test these contentions, we conducted an online vignette study (Study 1) and a field study (Study 2). In Study 1, we found a significant and negative main effect of ethical leadership on employees' uncertainty, but this effect was negated when leaders' close monitoring was high. In Study 2, we replicated the findings—ethical leadership was negatively related to employees' uncertainty and this relationship became nonsignificant when leaders' close monitoring was high. In addition, uncertainty was negatively related to employees' OCB toward the organization (OCBO) but not their OCB toward individuals (OCBI). Consequently, uncertainty mediated the relationship between ethical leadership and OCBO, and this mediation was moderated by close monitoring. However, such mediation and moderated mediation did not occur for OCBI. Taken together, our research reveals that close monitoring is not a viable strategy to be implemented alongside ethical leadership for managing employees' uncertainty and fostering their OCB.