This study examined the mediating effect of organizational commitment on the relationship between perceived organizational support and organizational citizenship behaviour in selected organizations within the Nigerian public sector. The study was motivated by the need to understand how supportive organizational practices influence employees’ willingness to engage in extra-role behaviours that enhance organizational effectiveness. A quantitative research design was adopted using a cross-sectional survey method. Data were collected from 250 employees through a structured questionnaire, and the responses were analyzed using descriptive statistics, correlation, regression, and mediation analysis. The findings revealed that perceived organizational support had a significant positive effect on organizational citizenship behaviour (β = 0.52, p < 0.05), indicating that employees who perceived greater support from their organizations were more likely to demonstrate discretionary behaviours beneficial to the organization. Perceived organizational support was also found to significantly predict organizational commitment (β = 0.61, p < 0.05), while organizational commitment significantly influenced organizational citizenship behaviour (β = 0.47, p < 0.05). When organizational commitment was introduced into the regression model, the direct effect of perceived organizational support on organizational citizenship behaviour reduced but remained significant (β = 0.28, p < 0.05), confirming a partial mediation effect. The indirect effect was statistically significant, suggesting that organizational commitment serves as an important mechanism through which perceived organizational support enhances citizenship behaviour. The study concludes that while perceived organizational support directly promotes organizational citizenship behaviour, its impact is strengthened when employees develop a sense of commitment to the organization. It is therefore recommended that public sector organizations in Nigeria adopt policies that enhance employee support, recognition, and welfare in order to foster commitment and encourage positive workplace behaviours that improve service delivery and organizational performance.