BACKGROUND: Multiple sclerosis (MS) has a relevant impact on quality of life (QOL) and is associated with increased risks of psychological morbidity. Mindfulness-based interventions (MBIs) are among the most studied interventions, although few well-conducted studies have tested them in this field. Furthermore, the participation in typical MBIs may be impaired by time and logistics. OBJECTIVE: We aimed to test the efficacy of an online MBI to improve QOL, psychological well-being, sleep, and fatigue. METHODS: We conducted a randomized controlled trial, in which 139 participants were randomly assigned to an MS-specific online mindfulness meditation intervention or to a psychoeducational (active control) group. Participants were assessed for QOL, depression, anxiety, sleep problems, and fatigue, at three different times: at recruitment, after 2 months, and after 6 months. RESULTS: In comparison to the control group, the experimental subjects reported higher QOL and lower depression, anxiety, and sleep problems at the end of intervention. However, after 6 months these group differences were no longer significant. CONCLUSION: An online MBI could be an effective psychological treatment for the promotion of well-being in MS in short-term. However, the lack of lasting effects requires the development of new strategies to support long-term changes.