Leaders occupy the most critical roles in organizations. They have to make decisions almost on a daily basis, and some decisions are challenging or unpopular. Feelings of regret can stem from looking back on past decisions and believing that a better outcome may have occurred if a different choice was made. Therefore, leaders could have higher chances of experiencing regret emotions and expressing their regret. However, the question of how leader’s expression of regret influences their perceived effectiveness has been left untouched by researchers and practitioners for years. Drawing on implicit leadership theory, through an experimental study with 374 participants, we showed that leaders who express regret are perceived as less effective. This study also revealed that the negative impact of leader regret expression on perceived leader effectiveness was mediated by observers’ perceived leadership prototypic violation. Moreover, we tentatively explored the leader gender’s role in the relationships. We found promising information showing that male leaders who expressed regret received the highest ratings of prototypic leadership violation and lowest ratings of leader effectiveness. In addition, regarding the female leaders, who are generally perceived as being too emotional for effective leaders, when they don’t express regret, they received the lowest ratings of prototypic leadership violation and highest ratings of leader effectiveness.