The rapid growth of mindfulness-based interventions (MBIs) has raised questions regarding their safety. This review aimed to quantify the adverse events of mindfulness-based stress reduction (MBSR) and mindfulness-based cognitive therapy (MBCT) reported in randomized control trials (RCTs) which included a statement of monitoring of adverse events. Literature was searched from the OVID databases until August 2017. A total of 36 RCTs were identified from 7931 search records, with 25 trials using MBSR and 11 trials using MBCT. A total of 19 (1%) and 19 (0.9%) adverse events were reported in the mindfulness intervention group and control group, respectively. In the MBSR group, none of them reported serious adverse event, and only three (12.0%) studies reported six (0.49%) intervention-related adverse events from among 1231 participants. No significant difference between the MBSR intervention and control groups was observed (6/1231 vs. 2/1244; risk difference (95% CI) = 0.0033 (− 0.01 to 0.01)). In the MBCT group, no intervention-related adverse events were reported, and only one trial reported ten (1.5%) cases of intervention-unrelated adverse events from 768 participants. No statistically significant difference in terms of the reported adverse events between the intervention and control groups was observed in the MBCT trials. Only a small proportion of trials reported monitoring of adverse events. Very few adverse events were reported in RCTs that used MBSR/MBCT. The MBSR/MBCT is regarded as relatively safe interventions. However, future studies are highly encouraged to report adverse events in mindfulness interventions for more affirmative conclusions.