Although previous literature has revealed the effect of a single social identity on trust, only few studies have examined how multiple social identities affect trust in others. The present research examined the effects of trustors' social identity complexity on their level of trust toward another person (interpersonal trust), outgroup members (outgroup trust), and ingroup members (ingroup trust). Study 1, which was a correlational study, indicated that trustors' social identity complexity was positively related to their interpersonal and outgroup trust. Three experimental studies were performed to identify causal relationships. Study 2 found that activating trustors' high social identity complexity produced high levels of interpersonal trust, and Studies 3 and 4 found that this effect was more pronounced when the trustee was an outgroup member (outgroup trust) rather than an ingroup member (ingroup trust). The implications of these results for social harmony are discussed.