Blood donation is an acutely prosocial behaviour – an act that literally involves giving a part of oneself. Negative emotions are notoriously implicated as barriers to donation. In this research, we turn focus towards positive emotions, seeking to identify their role in motivating individuals to donate blood. Based on analyses of a range of data from more than 2000 participants collected in donor centres, in online longitudinal and cross-sectional surveys, and in virtual-reality simulations of donation, conclusions consistently point to the motivating force of positive emotion. Specifically, our findings reveal that that individuals who experience positive emotion during blood donation or who anticipate experiencing positive emotion if they were to donate are more willing to engage in, and actually do engage in, this prosocial act. Our work also explores how such findings might be leveraged to promote donation, namely via emotion-regulation based techniques aimed towards promoting experienced and anticipated positive emotion. The insights afforded by adopting an empirical focus on positive emotion in the blood donation context pave a path for both practical and theoretical advance.