Patients with major depressive disorders (MDD) exhibit social dysfunction, as illustrated by the lower acceptance rate of unfair proposals in the Ultimatum Game (UG) among patients with MDD compared with a control group. However, the underlying mechanisms remain largely unknown. In this study, we explored whether patients with MDD had altered perceptions of fairness or altered perception-behavior linkages compared with the control group, using a multilevel moderated mediation framework. Sixty-eight unmedicated patients with MDD and 55 members of a control group were recruited. Using generalized linear mixed effects models and multilevel structural equation modeling, we investigated the differences in the linkages between fairness level, fairness perception, and acceptance behavior among the two groups playing the UG. The results showed that the patients with MDD had a lower acceptance rate of unfair proposals than the control group. Fairness perception mediated the relationship between fairness level and acceptance behavior for both groups of participants when they played with human proposers but not computer proposers. The mediation effect was stronger among the control group than among the MDD patients. The linkage between fairness perception and acceptance behavior was attenuated among the patients with MDD compared with the control group. In conclusion, MDD patients were impaired in their ability to flexibly adjust acceptance behavior based on fairness perception in social interactions. (PsycInfo Database Record (c) 2022 APA, all rights reserved).