We test whether a brief mindfulness induction reduces self‐serving bias in bargaining. In a pre‐registered online experiment ($N=543$), participants were randomly assigned to an eight-minute mindfulness exercise or a mind-wandering control before playing a one-shot Nash demand game with asymmetric stakes. A prior dictator game profiles “selfish” types. Mindfulness has no average effect on claims, but its effects are highly heterogeneous: among selfish participants in the disadvantaged role, it markedly tempers over-claiming, lowers the probability of disagreement, and raises joint expected payoffs. Elicited fairness benchmarks are essentially unchanged across treatments (a 50/50 split remains the average), yet mindful selfish participants align behavior more closely with their stated fairness, consistent with enhanced self-regulation rather than preference change. The results provide causal, incentive-compatible evidence that a light-touch attentional intervention can attenuate self-serving bias and improve efficiency in strategic interaction, with benefits concentrated among those most prone to biased claims.