Objectives The current study assessed whether trait mindfulness relates to social decision making as increased acceptance rates towards offers in the Ultimatum Game. Mindfulness has been associated to a reduction in emotional reactivity and an increase in emotion regulation once intense emotions do occur. Therefore, we reasoned that trait mindfulness would predict acceptance in the Ultimatum Game, perhaps even in case of unfair offers. Methods In two online studies we assessed whether trait mindfulness positively predicts acceptance of offers in the Ultimatum Game among community samples (study 1 N = 107; study 2 N = 118). In study 2, we also assessed participants’ emotional reactions to offers prior to their decision to accept or reject. Results Whereas study 1 indeed showed a significant positive relation between trait mindfulness and acceptance of offers (OR = 2.01, p = .05), study 2 did not show this relation (OR = .91, p = .81). Also, the results of study 2 showed that trait mindfulness may moderate emotional responses to offers (β =  − .06, p = .03). Yet, analyses of the pooled data indicated no relation between trait mindfulness and acceptance of offers (p < .15). Conclusions Our research provides mixed support regarding the association between trait mindfulness and behavioral acceptance of offers in the Ultimatum Game. We discuss the need for more fine-grained examinations of when and why mindfulness should lead to acceptance of unfairness, and if and when mindfulness would lead to wise responding in social exchange situations.