Non-suicidal self-injury (NSSI), the purposeful damaging of one’s own body tissue (e.g., cutting), represents a common, yet highly stigmatized issue among university students. When individuals disclose NSSI and are met with stigmatizing responses, this can decrease the likelihood that they will seek further support, including from professionals. To address this, researchers have highlighted the need for widespread training in effective responding to NSSI disclosures for key university campus stakeholders who may be in a position to receive such disclosures. The current study developed, implemented, and evaluated such a training for a campus peer support group, which focused on psychoeducation about NSSI and evidence-informed best practices. Evaluation of the training was conducted using a mixed-methods approach based on the Kirkpatrick model, which involved assessment at four separate levels: reactions (e.g., did participants find the training useful), learning (e.g., increases in knowledge about NSSI, more positive attitudes about those who self-injure), behaviour (e.g., improvements in intended response to disclosures), and results (longer-term maintenance of change at the learning and behaviour levels). Thirty-two undergraduate peer support volunteers took part in a pre-training questionnaire, with 17 of these individuals completing at least part of a post-training questionnaire, and 9 completing at least part of a three-month follow-up. When comparing pre-training to post-training, both quantitative and qualitative gains in perceived knowledge, intended behaviour, and self-efficacy in responding to disclosures were evident. Additionally, at least some of these gains appeared to have been maintained at follow-up. This suggests that short, one-session, practical trainings for peer support volunteers on responding to NSSI disclosures appear promising, although more research is needed.