Overconfidence is defined as the difference between the average estimated certainty of one’s decision and the average accuracy of the decision, that is, the percentage of correct answers of the individual. The aim of this research was to examine the relationship between overconfidence in decision-making with personality traits and cognitive reflexivity. 304 participants took part in this research of which 177 (58.2%) were women. The research was conducted online via Google forms in which overconfidence was examined with the General Knowledge Questionnaire in which after every question, participants had to assess their confidence in the accuracy of their answer. Personality traits were examined with the Big Five Inventory, and cognitive reflection was examined with the Cognitive Reflection Test. The results indicate that personality traits (extraversion, agreeableness, conscientiousness, neuroticism, openness to experience) are not associated with overconfidence in decision-making. A significant negative correlation was found between cognitive reflexivity and overconfidence in decision-making, and cognitive reflexivity also proved to be a significant negative predictor of overconfidence. Cognitive reflexivity explained 5.6% of the variance of overconfidence. That is, cognitively reflective people tend to be less overconfident in their decisions than cognitively non-reflective people. The importance of this research lies in the ability to recognize overconfidence in decision-making in everyday life and to prevent its negative consequences.