BACKGROUND: Evidence suggests that facilitating empathy could improve individuals' well-being. Loving-kindness meditation (LKM) could be a facilitator, and online delivery a cost-effective format. METHODS: We conducted an internet-based randomised controlled trial recruiting 809 adults to test whether an LKM course improves well-being through evoking pleasant emotions, psychological resources, and altruism compared to a light physical exercise course (LE). Participants in both arms followed video-based instructions, completed post-intervention questionnaires, and used online diaries and forums. To measure altruism £10/$10 were offered to participants with a choice of donating all/half to charity. Thematic analysis was applied to diary/forum entries. RESULTS: Both courses increased well-being without significant differences. LKM participants were less anxious than LE participants (ß = -0.22, 95% confidence interval (CI) [-0.43, -0.02], p = .03), and more likely to donate £5/$5 (Relative Risk = 3.57, 95%CI [0.82, 15.50], p = .09). Attrition was high (82%). Participants engaged in diary/forum usage. LKM was an emotionally intense experience, generating deep reflections and increased connectedness but difficult for some to process. LE led to gentle increases in relaxation, generating a sense of achievement. CONCLUSIONS: Future research needs to confirm findings and devise ways of delivering online LKM effectively to diverse populations.