Background: While considerable attention has been given to explanations for youth suicide, less is known about the reasons that young people themselves give for suicidality. Research on online communications gives an opportunity to investigate the real-time reasons young people give for feeling suicidal. Aims: This study aimed to identify the reasons that young people provide for feeling suicidal in posts published on a suicide prevention forum, hosted on the social media platform Tumblr. Method: We filtered 2 months' worth of posts to identify those that related specifically to suicide. In total, 210 posts were thematically analyzed to identify the reasons given for suicidality and the meanings associated with these. Results: Six main reasons for suicidality were identified in the analysis: feeling lonely and socially disconnected, experiencing identity stigma, failing to meet expectations, being helpless, feeling worthless, and experiences of mental ill-health. Limitations: There are advantages as well as limitations associated with relying on Internet-based data. Limitations include the inability to establish participant demographics and the lack of context for posts. Conclusion: Suicide prevention efforts should target the reasons that young people give for feeling suicidal in the moment of crisis in order to engage this population more effectively.