Young people may be particularly vulnerable to the mental health impacts of the COVID-19 pandemic and may also be more likely to use social media at this time. This study aimed to explore young people's mental health and social media use during the COVID-19 pandemic and examined their use of social media to seek and provide support for suicidal thoughts and self-harm during this period. Young people aged 16-25 (n = 371, M = 21.1) from the general population in Australia completed an anonymous, cross-sectional online survey advertised on social media from June to October 2020. Participants reported high levels of psychological distress, with over 40% reporting severe levels of anxiety and depression, and those with a mental health diagnosis were more likely to perceive the pandemic to have had a negative impact on their mental health. Gender-diverse participants appeared the most negatively impacted. Social media use was high, with 96% reporting use at least once a day, and two-thirds reporting an increase in social media use since the start of the pandemic. One-third had used social media to seek support for suicidal thoughts or self-harm, and half had used it to support another person. This study adds to a growing literature suggesting social media can provide an opportunity to support young people experiencing psychological distress and suicide risk. Uniquely, this study points to the utility of using social media for this purpose during high-risk periods such as pandemics, where access to face-to-face support may be limited. To promote the quality and safety of support provided on social media, resources for help-seekers and help-givers should be developed and disseminated. Social media companies must consider the vulnerability of some users during pandemics and do what they can to promote wellbeing and safety.