BackgroundEvidence for the longer-term benefits of online mindfulness training for medical students, including in the reduction of stress and improved wellbeing, is limited. This study aimed to evaluate the impact of a novel online mindfulness training program on trait mindfulness, wellbeing and study engagement of medical students at program completion and 6-month follow-up.MethodsThis was a randomised waitlist control study of an 8-week, online, mindfulness-based intervention versus normal curriculum alone for medical students (N = 114). The primary outcome measures were the changes from baseline to program completion at Week 8 for self-reported trait mindfulness (Freiburg Mindfulness Inventory), perceived stress (Perceived Stress Scale), self-compassion (Self-Compassion Scale) and study engagement scores (Utrecht Work Engagement Scale for Students). The secondary outcome measures were these score changes from baseline to 6-month follow up. Intervention and control students completed surveys at all three time points. Program adherence (Mindfulness Adherence Questionnaire) was also measured in the intervention group.ResultsThe intervention group experienced modest but statistically significant improvements in mindfulness (9%, p = 0.0002), self-compassion (5%, p = 0.026), and study engagement (4%, p = 0.035) from baseline to Week 8. They also reported a sustained improvement of 5% (p = 0.017) in mindfulness scores at 6 months. The control group reported no significant changes at Week 8 or 6 Months. Between-group comparisons showed that compared to the control group, the intervention group improved significantly more in mindfulness (p = 0.0076), and statistically marginally more in study engagement (p = 0.0534) at Week 8. No statistically significant between-group differences were observed at 6 months.ConclusionsThese results add to the small but growing body of evidence suggesting that online mindfulness-based interventions with minimal contact can improve, albeit in modest magnitude, mindfulness and possibly study engagement in medical students for the duration of a mindfulness program. Further refinements to the program may be important to maintain improvements in the longer-term.Trial registrationRegistration number ACTRN12624000783527.