Fairness is a key concept in social interactions and is influenced by intentionality considerations. In this functional magnetic resonance imaging study, we investigated the neural correlates of fairness by focusing on responder behavior to unfair offers in an Ultimatum Game paradigm with conditions that differed in their intentionality constraints. Brain activity underlying rejection vs acceptance of unfair offers appeared highly dependent on intentionality. Rejection of unfair offers when the proposer had no-alternative as well as acceptance of offers when the proposer had a fair- or hyperfair-alternative was associated with activation in a network of regions including the insula and the dorsal medial prefrontal cortex. These activations were interpreted as neural responses to norm violations because they were mostly involved when behavior was inconsistent with socially accepted behavior patterns. Rejection of unfair offers in the no-alternative condition further resulted in activity in the anterior medial prefrontal cortex and the temporoparietal junction, which was interpreted in terms of higher moral mentalizing demands required in social decision-making when rejection could not be readily justified. Together, results highlight the significance of intentionality considerations in fairness-related social decision-making processes.