Intranasal administration of the neuropeptide oxytocin (OT) appears to have positive social consequences, but these effects are often highly context- and person-specific. The present research examined whether the core personality trait of extraversion may be one important person-specific factor that plays a role in these associations. Across two double-blind randomized placebo-controlled studies (total ns: Study 1 = 121; Study 2 = 112), we observed significant interactions between OT administration and extraversion predicting prosocial outcomes. For individuals low in extraversion, OT administration relative to placebo led to greater perceived social connection and prosocial tendencies (Study 1) and more positive behavioral responses to help and greater trust of an interaction partner (Study 2). In contrast, OT administration was not beneficial for individuals high in extraversion. Overall, these findings contribute to growing evidence that OT administration has complex, person-specific effects on social behavior, indicating that extraversion plays an important role in these associations.