Four studies tested the proposition that mindfulness and its training fostered prosociality toward ostracized strangers. In discovery Study 1, dispositional mindfulness predicted greater empathic concern for, and more helping behavior toward, an ostracized stranger. Using an experimental design, Study 2 revealed that very briefly instructed mindfulness, relative to active control instructions, also promoted prosocial responsiveness to an ostracized stranger. Study 3 ruled out alternative explanations for this effect of mindfulness, showing that it did not promote empathic anger or perpetrator punishment, nor that the control training reduced prosocial responsiveness toward an ostracized stranger rather than mindfulness increasing it. Study 4 further ruled out the alternative explanation of relaxation in the experimental effects of mindfulness. In all studies, empathic concern mediated the relation between mindfulness and one or both of the helping behavior outcomes. Meta-analyses of the four studies revealed stable, medium sized effects of mindfulness instruction on prosocial emotions and prosocial behavior. Together these findings inform about circumstances in which mindfulness may increase prosocial responsiveness, and deepen our understanding of the motivational bases of prosociality.