Adolescents often look to their peers for emotional support, so it is critical that they are prepared to take on a supportive role, especially during a health crisis. Using a randomised controlled trial (ISRCTN99248812, 28/05/2020), we tested the short-term efficacy of an online training programme to equip young people with skills to support to their peers’ mental wellbeing during the COVID-19 pandemic. In June 2020, one-hundred UK adolescents (aged 16-18) recruited through social media were randomly allocated (1:1) to immediate 5-day peer support training or a wait-list, via an independently generated allocation sequence. Primary outcomes were indicators of ability to help others (motivation, perceived skills, frequency of help provided, compassion to others and connectedness to peers). Secondary outcomes included emotional symptoms, mental wellbeing, and indicators of agency (civic engagement and self-efficacy). We also collected qualitative reports of participants’ experience. Assessments were completed at baseline and 1 week post randomisation (primary endpoint), and up to 4 weeks post randomisation (training group only). The training increased support-giving skills, frequency of providing support, compassion and peer connectedness (medium-large-effect sizes), but not motivation to provide support, 1 week post randomisation, compared to controls. Gains in the training group were maintained 4 weeks post randomisation. Training also improved adolescents’ mental health and agency, and qualitative reports revealed further positive outcomes including increased self-care and empowerment. Leveraging digital platforms that are familiar to young people, peer support training has the potential to enable adolescents to support their own and their peers’ mental wellbeing during a health crisis.