Aim: Employee job engagement is a critical determinant of organizational performance and effectiveness of organizations and is shaped by both individual characteristics and organizational factors. This study investigates the effect of proactive personality on job engagement by examining the mediating roles of thriving at work and job flourishing, as well as the moderating role of perceived organizational support. Methods: The research is applied in nature and employs a descriptive-correlational method survey design. The study population comprised 250 nurses employed at a public hospital in Tehran, Iran, of whom 176 were selected using Cochran's formula for finite populations. Data were collected using validated questionnaires and analyzed through Structural Equation Modeling based on partial least squares (PLS-SEM) using SmartPLS 4 and SPSS 27 software. Results: The findings indicate that proactive personality has a significant positive direct effect on job engagement. Furthermore, proactive personality indirectly enhances job engagement through thriving at work and job flourishing. However, perceived organizational support does not significantly moderate the relationships between proactive personality and either thriving at work or job flourishing. Conclusion: The findings suggest that fostering proactive personality and creating a work environment that promotes thriving at work and job flourishing can enhance employees’ job engagement. Furthermore, the non-significant moderating effect of perceived organizational support indicates that, within the study context, the influence of proactive personality on job engagement is driven more by employees’ individual characteristics and personal resources than by their perceived organizational support.