Background: Mindfulness-based interventions (MBIs) have been reported to be efficacious in treating attention deficit hyperactivity disorder (ADHD). However, the value of the control effect of MBIs on ADHD core symptoms remains controversial. To clarify the literature on the control effect of MBIs on the symptoms of ADHD and guide future researches, an effect-size analysis was conducted. Methods: A systematic search in PubMed, Embase, Web of Science, Medline, Cochrane Library, China National Knowledge Infrastructure, and Wangfang Data databases was performed up to January 11, 2019. The overall effect size of MBIs on ADHD core symptoms (ie, inattention and hyperactivity/impulsivity) was recorded by the metric of Hedges' g with 95% confidence interval, Z-value, and P-value. Results: Eleven eligible studies featuring 682 participants were included in the meta-analysis. The overall results indicated that MBIs had large effects on inattention (Hedges' g = -0.825) and hyperactivity/impulsivity (Hedges' g = -0.676) relative to the control group. Results from subgroup analyses between self- and observer rating on ADHD symptoms revealed that the effect of MBIs both remained in a large range and self-rated ADHD core symptom had a greater impact on heterogeneity across the studies. Meta-regression found that the overall effect might be moderated by participant age group and control condition. Conclusion: The present meta-analysis suggested that MBIs had large effects in reducing ADHD core symptoms in comparison with the control group. Future researches are needed to assess follow-up effects of MBIs on ADHD core symptoms and explore the correlation between the individual level of mindfulness and reduction of ADHD symptoms.