Millions of people who could benefit from mental health services do not receive treatment. If non-professional peers could learn to administer basic psychotherapeutic interventions to each other, taking turns as care provider and care recipient, this unmet need for mental health care could be partially filled. This study sought to test whether non-professionals could learn supportive psychotherapy skills from a massively scalable, free online course. Thirty pairs of individuals who were experiencing psychological distress or who wished to increase their mental well-being were enrolled in the study, and 19 pairs completed the prototype online course. Objective raters assessed participants’ skills usage before and after the course by coding ecologically valid video recorded stressor discussions. Participants increased their use of supportive psychotherapy skills as a result of taking the course, and they decreased potentially harmful behaviors. Forty-three percent of those who completed the course met criteria for competency to deliver the intervention. Participants believed that the discussions they had after taking the course were more helpful than those they completed prior to training. Though the course will require refinement, and the effects of the intervention on mental health will need to be tested before it is released to the general public, this study suggests that training non-professionals to deliver reciprocal peer support interventions via free online courses holds promise as an accessible strategy for alleviating the burden of psychological distress.