BACKGROUND: Psychological interventions are commonly applied in sports to help athletes enhance their performance, but the effect psychological interventions have on actual performance is unclear despite decades of research. OBJECTIVE: We conducted a systematic review with meta-analyses to investigate the effects of a wide range of psychological interventions on performance in competitive athletes. METHODS: A study protocol was preregistered in PROSPERO, and a literary search was performed in the MEDLINE, PsycINFO, Web of Science, and SPORTDiscus databases. Psychological intervention studies were eligible by using a group design and a quantitative performance outcome with athletes competing at a regional or university level or higher. Included studies were assessed regarding intervention characteristics, research methodology, and risk of bias. A multi-level meta-analysis framework with cluster robust variance estimation was used to quantitatively synthesize the results. RESULTS: A total of 111 studies met the inclusion criteria, and 25 of these studies (37 effects) could be synthesized into five meta-analyses in which there were similarities in the type of psychological intervention, comparator, and experimental design. Meta-analyses I (multimodal psychological skills training vs control), II (mindfulness- and acceptance-based approaches vs control), and III (imagery vs control) consisted of parallel-group studies, and random-effects models were used to calculate the standardized mean difference. Meta-analyses IV (attentional focus strategies, external vs internal) and V (regulatory focus performance instructions, prevention vs promotion) consisted of counterbalanced crossover design studies, and random-effects models were used to calculate the standardized mean change using change score standardization. Significant results were found in three of the meta-analyses (I, II, and III). Psychological skills training (g = 0.83, 95% confidence interval 0.21-1.45), mindfulness- and acceptance-based approaches (g = 0.67, 95% confidence interval 0.01-1.32), and imagery (g = 0.75, 95% confidence interval 0.14-1.36) outperformed controls with moderate effects. However, when non-randomized trials and subjective performance outcomes were removed in sensitivity analyses, the overall estimates of the effect size were no longer significant in any of the syntheses. CONCLUSIONS: The significant moderate effects for psychological skills training, mindfulness- and acceptance-based approaches, and imagery are not stable, and further trials with robust research methodology, such as randomized controlled trials, are requested for all types of psychological interventions aiming to enhance performance in athletes. Moreover, improved reporting standards and the provision of datasets in open science repositories are important to consider in future trials in sport psychology. CLINICAL TRIAL REGISTRATION: PROSPERO CRD42017056677.