Job satisfaction (JS) is an indicator of individual psychosocial health. Consistent evidence showed that voluntary extra-role behavior in organizations, namely organizational citizenship behavior (OCB), can also contribute to individual psychological health. JS has been found to positively influence employees’ OCB, and both JS and OCB have been found to predict employees’ task performance (TP). The purpose of this study was to investigate whether employees’ OCB mediates the relationship of JS with TP, taking into consideration gender as a potential moderator, and other sociodemographic and work-related characteristics as confounding variables. A total of 518 employees, 54.6% women, aged 19–66 years with a mean age of about 36 years, completed measures of JS, OCB, and TP. Results showed a partial mediation of OCB in the JS-TP relationship, which was invariant across gender. A potential practical implication of findings is that human resource managers and practitioners might ultimately benefit male and female employees’ well-being as well as the organizations’ productivity by developing targeted individual- and group-level trainings and interventions to enhance JS and OCB.