Background <p>Interprofessional collaboration between physicians and pharmacists is important for ensuring medication safety. Community pharmacists play a vital role in this process through prescription recommendations to physicians. Assertiveness, defined as a communication style that attempts to enhance mutual understanding while respecting both oneself and others, is recognized as useful for making prescription recommendations. Based on the concept of assertiveness, “functional assertiveness,” which focuses on achieving tasks while maintaining interpersonal relationships, has been proposed and the Functional Assertiveness Scale (FAS) was developed to evaluate it. Although functional assertiveness has been reported to be useful for achieving the task of ensuring medication safety while maintaining professional relationships with physicians, its applicability to community pharmacists has not been verified. This study aimed to investigate the reliability and validity of the FAS for Japanese community pharmacists and evaluate its utility.</p> Methods <p>A cross-sectional study was conducted using an online questionnaire targeting 2,190 community pharmacists between July and September 2025. Structural validity was examined using exploratory factor analysis (EFA) and confirmatory factor analysis (CFA). Internal consistency was assessed using Cronbach’s α coefficient. Construct validity was verified by calculating Spearman’s rank correlation coefficients between FAS scores and the Assertive Self-expression (AS) score of the Interprofessional Assertiveness Scale.</p> Results <p>Analysis was performed on 415 participants (mean age 41.0 years; 54.9% women). EFA confirmed a two-factor structure consistent with the original scale. CFA showed good model fit based on SRMR (0.048); however, CFI (0.893) and TLI (0.867) were slightly below conventional thresholds. Cronbach’s α for the total FAS was 0.85, indicating good internal consistency. Construct validity was supported by a significant positive correlation between FAS total score and AS score (rho = 0.34, <i>p</i> &lt; 0.01).</p> Conclusions <p>FAS demonstrated sufficient reliability and a clear two-factor structure for application among Japanese community pharmacists. Our findings suggest that FAS serves as a useful tool for objectively measuring functional assertiveness among community pharmacists. On the other hand, the CFA results indicate the need for future item optimization specifically tailored to pharmacy practice. Further studies are needed to develop a pharmacist-specific version to contribute to evaluating educational programs and enhance interpersonal communication in pharmacy practice.</p>