This study was conducted to establish foundational data for educating and assessing medical professionalism in medical students. The study aimed to validate a list of unprofessional behaviors and evaluate how medical school faculty perceive the importance of these behaviors in the assessment of professionalism. Content validation was performed by six medical education experts on 38 unprofessional behavior items developed based on a literature review, resulting in the confirmation of 32 items. Subsequently, the perceived importance of these items was surveyed among 100 medical faculty members, followed by exploratory factor analysis (EFA), group mean comparisons, and repeated measures ANOVA. The EFA identified three factors comprising a total of 17 items: ‘Lack of Responsibility and Insight,’ ‘Dishonest Behavior,’ and ‘Interpersonal Disrespect.’ While no significant differences in perceptions were found based on faculty characteristics (gender, age, specialty, or duration of teaching), significant differences were observed in the relative importance among the three factors. Faculty members rated ‘Dishonest Behavior’ as the most critical, followed by ‘Disrespectful Behavior’ and ‘Lack of Responsibility and Insight.’ These findings suggest that international classification systems for unprofessional behaviors can be validly applied within the Korean medical school context, particularly highlighting that faculty evaluate overt rule-breaking more strictly than a lack of internal reflection. Therefore, future professionalism education should pair the provision of rigorous guidelines for dishonest or disrespectful behaviors with educational interventions focused on professional identity formation to cultivate the underlying responsibility and insight that form the basis of medical professionalism.