Bargaining parties often disagree on what fair is, due to the reason that people are prone to believe that what favors oneself is fair, i.e., an egocentric bias. In this study, we investigated the neural signatures underlying egocentric bias in fairness decision-making, conjoining an adapted ultimatum game (UG) with event-related fMRI and functional connectivity. Participants earned monetary rewards with a partner in a production stage, wherein their contributions to the earnings were manipulated. Afterwards, the joint earnings were randomly divided, and the distribution was presented simultaneously with contribution information to participants, who accepted/rejected distributions of earnings as the same manner in standard UG. We identified an egocentric bias in fairness decisions, such that participants frequently rejected self-contributed disadvantageous outcomes, but much less so in response to other-contributed advantageous outcomes, although both involved mismatch between contribution and payoff. This bias was underpinned by regions involved in representing fairness norms, including the anterior insula and dorsal anterior cingulate cortex (dACC). Furthermore, the thalamus activity was predictive of the bias, such that the level of egocentric bias decreased as a function of the activation level of the thalamus. Finally, our functional-connectivity findings indicated that the thalamus worked together with insula and dACC to modulate behavioral egocentric bias in fairness-related decisions. Our findings uncover the neural basis underlying the modulation of egocentric bias in normative decision-making, and highlight the role of neural circuits associated with norm enforcement in this phenomenon.