PurposeThis research paper aims to elucidate why and how a fair supervisor influences an employee's job satisfaction. While various theoretical approaches have been explored and numerous explanatory mechanisms investigated in prior organizational justice research, it is still unclear which explanatory mechanism is the dominant one to explain fairness effects. To address this gap, the author compares six distinct explanatory mechanisms of fairness effects on job satisfaction.Design/methodology/approachThe author conducted a three-phase survey study with 309 employees from diverse organizations. The author measured all variables twice to control for stability effects and ensure stable findings. The author combined a path analysis with bootstrapping procedures using Mplus 8.3 software.FindingsThe influence of supervisor fairness on job satisfaction is primarily transmitted through an employee's negative emotions, a mechanism often examined in previous organizational justice research adopting the moral perspective of fairness.Practical implicationsSupervisors can increase employees' satisfaction with their jobs by treating them fairly and promoting a fair work environment. To increase the benefits of workplace fairness, supervisors can focus on the intervening mechanisms, such as emotions.Originality/valueFirst, the author provides a fine-grained understanding of why supervisor fairness increases job satisfaction. Second, the author clarifies how the effects of supervisor fairness are transmitted. Third, the author identifies the most critical mediator to explain how supervisor fairness affects job satisfaction.