Organisational Citizenship Behaviour (OCB) is crucial to improving organizational performance in the maritime industry. This study sought to investigate the direct and indirect effects of leadership style (transformational vs. transactional), workload (number of hours worked per day), and training type (on-the-job vs. classroom) on organizational citizenship behavior (OCB) via perceived supervisor support as a mediator. Also serving as a moderator between perceived supervisor support and OCB was employee job tenure. Data were obtained from Indonesian Seafarers using cluster sampling techniques. Multiple regression analyses revealed that transformational leadership, lower burden, and on-the-job training positively influenced OCB, while perceived supervisor support mediated the indirect effect of all three IVs. In addition, job tenure moderated the relationship between supervisor support perceptions and OCB. The research has significant implications for the maritime industry’s efforts to improve OCB among its employees. In addition, this study contributes to the literature by investigating the roles of perceived supervisor support and job tenure in the relationship between leadership style, workload, training type, and OCB among Indonesian Seafarers.