Objectives <p>Trait mindfulness is associated with many measures of individual well-being, but its relationship to prosocial behavior is less clear. Prior research found that a brief intervention boosting <i>state</i> mindfulness led to increased prosocial behaviors among individuals with interdependent self-construals, but decreased prosocial behaviors among individuals with independent self-construals. The present research sought to examine <i>trait</i> mindfulness and prosocial behavior and to examine the moderating roles of both horizontal and vertical interdependence.</p> Methods <p>Participants (<i>n</i> = 149) came to a lab and read about a charitable cause. They then had the opportunity to stuff envelopes on behalf of a fundraising appeal for that cause. Previously, outside of the lab, participants had completed measures of trait mindfulness, self-construal, and individualism-collectivism.</p> Results <p>Trait mindfulness predicted increased helping behavior in the form of stuffing envelopes among people high in collective interdependent self-construal and among those low in horizontal or vertical individualism.</p> Conclusions <p>Findings suggest that trait mindfulness can predict either greater or lesser prosocial behavior depending on people's preexisting social goals and identities, and that this pattern is not limited to vertical individualism.</p> Preregistration <p>This study is not preregistered.</p>