Introduction: Psychotherapies, such as mindfulness-based interventions (MBIs), are currently needed to tackle mental health problems. Online MBIs have become promising since face-to-face interventions are limited during the COVID-19 pandemic due to lockdown and social distancing. This systematic review and meta-analysis aimed to investigate the effect of online MBIs in improving mental health, mainly depression, anxiety, and stress.

Materials and methods: A systematic literature search was conducted according to the PRISMA 2020 guidelines on several databases for eligible studies up to October 17, 2021. Study quality was assessed using the Cochrane's Risk of Bias 2 tool. Effect sizes were presented as standardized mean difference (Hedges' g) between the online MBIs and control groups at post-test and follow-up using a random-effects model.

Results: Eight randomized controlled trials involving 868 participants were included in this meta-analysis. The pooled adherence rate to online MBIs was 94% (95% CI = 91% to 98%). The findings revealed that online MBIs had a statistically significant small to moderate effect in reducing depression (g = -0.32; 95% CI = -0.49 to -0.14; I2 = 0%), a small effect on anxiety (g = -0.25; 95% CI = -0.43 to -0.06; I2 = 27%), and a moderate effect on stress (g = -0.62; 95% CI = -1.09 to -0.16; I2 = 83%). In addition, significant small effects at follow-up were observed for depression (g = -0.26; 95% CI = -0.48 to -0.04; I2 = 0%) and anxiety (g = -0.28; 95% CI = -0.48 to -0.08; I2 = 0%), but not for stress.

Conclusion: Online MBIs have beneficial effects on mental health, particularly depression, anxiety, and stress, during the COVID-19 pandemic. Given the limitations of the current study, future trials that specifically consider potential effect influencing factors, longer follow-up evaluation, and methodological quality are warranted.