Evidence shows that self-disclosure plays an important role in developing and maintaining close interpersonal relationships. As self-disclosure remains largely unexamined in the context of interventions based on youth-adult helping relationships, little is known about the effects of mentor disclosure, or the ethics of using this communication technique. In this study, we used selfreport questionnaire data from 51 mentoring pairs to investigate the effect of mentor self-disclosure on relationship quality in youth mentoring relationships, and consider the ethical challenges that arise when helping adults disclose to young people. Bivariate correlations showed mentor self-disclosure was significantly associated with relationship quality for mentees, but not mentors. Qualitative content analysis showed mentors were aware of how their disclosure may have ethical implications associated with the age and role-appropriateness of topics, contradictions between their own and the mentees’ family or cultural values, and the potential to negatively influence mentee behaviour. We consider these findings in a context of ethics in youth mentoring to raise questions about the intersection of disclosure, relationship quality, and safe mentoring practice.