Objective: Problematic Internet and smartphone use is associated with serious mental health problems. We thus investigated the effectiveness of psychological interventions on problematic Internet and smartphone use. Methods: We conducted a systematic review and meta-analysis and searched in PubMed, PsyArticles, PsycInfo, Medline and Google Scholar for peer-reviewed experimental and quasi-experimental research published between 2008 and 2021. Results: A total of 10 studies accompanied by 14 effect sizes were included in the meta-analyses for problematic Internet use and 3 studies accompanied by 4 effect sizes for problematic smartphone use. The total sample size was n = 1439 with a mean age of 20.34 (SD = 6.63) years. Psychological interventions were effective for both problematic Internet use ( g = −1.41, 95% confidence interval = [−1.90, −0.91], p < 0.001) and problematic smartphone use ( g = −0.40, 95% confidence interval = [−0.79, −0.01], p < 0.001). Conclusion: Current evidence indicates that psychological interventions can be effective in reducing problematic Internet use/problematic smartphone use. However, the results must be considered preliminary due to the limited number of studies available in the respective fields.