Background <p>The mental health of young people is a major public health concern and there is a high demand for support, but available mental health resources are limited. To address this problem, online peer support has potential to complement traditional approaches to providing mental health services. While some studies have identified possible mechanisms of peer support, there is limited understanding regarding the potential mechanisms underpinning mental health peer support delivered online. The aim of this systematic scoping review was to identify and summarise the mechanisms that have been proposed to explain why young people may use and benefit from online mental health peer support from the perspective of both the participants themselves and the external characteristics of online communities. The review also explored possible negative aspects of mechanisms and how associated risks may be managed.</p> Methods <p>The IBSS, SSCI, Scopus, PsycINFO, Medline and Social Policy and Practice databases were searched to identify relevant literature. Retrieved studies (<i>n</i> = 7495) were double screened and 24 articles met the criteria to be included in the review and analysed to identify key themes.</p> Results <p>Five main proposed theoretical mechanisms were found to underpin effective online peer support for young people with mental health issues: unidirectional interaction, one-way pointing to help-seekers from peer supporters and reciprocal (mutual) interaction between peer supporters and help seekers, which was the main interaction; knowledge about mental health including experiential knowledge; understanding which includes self-understanding, understanding others and feeling understood by others; social support; and the characteristics of the online community environment. Negative aspects associated with the challenges of online mental health peer support were also identified.</p> Conclusions <p>The identified, proposed theoretical mechanisms offer a better understanding of online peer support for youth mental health. They may explain why some young people choose online mental health peer support and they provide professionals and developers with insights for designing digital intervention tools. Future research is needed to explore the relative effectiveness of the proposed mechanisms, and the components within them. This could include comparing the effectiveness of online and offline peer support, and examining how these various mechanisms may contribute and combine to achieve maximum effectiveness.</p>