The study aims to explore the relationship between firm size and perceived organizational support (POS) and to investigate how supervisor support might moderate this relationship. The study involved a cross-sectional questionnaire survey in Japan with 25,648 participants. Assessing POS through eight-question. Firm size was categorized into small (2-49 employees), medium (50-999), and large (1000 or more). Coefficients were estimated using multiple regression analyses. The analysis revealed that medium and large firms had lower POS compared to small firms, with the difference being more pronounced in medium firms. A positive interaction between firm size and supervisor support was observed for POS. This study found that although POS is more likely to be lower in medium and large firms than in small firms, the effect of supervisor support on improving POS is stronger.