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.