In recent years, mindfulness-based interventions (MBIs) have been widely applied in competition sports with respect to athletic performance and mental health promotion, whereas evidence of randomized controlled trials (RCTs) has not been well summarized. Therefore, this study aimed to systematically review and meta-analyze the existing evidence on the effects of MBIs on improving athletic performance, mindfulness level, mindfulness-related psychological components (e.g., acceptance, self-compassion, flow), and mental health (e.g., burnout, stress, psychological well-being) among athletes. Following the PRISMA guidelines, a literature search was implemented on five electronic databases (Web of Science, PubMed, Scopus, ProQuest, and ScienceDirect) and relevant review papers. The article selection, risk of bias assessment, and data extraction were performed by two investigators independently. The standardized mean difference (SMD) was calculated to evaluate the effects of interventions using the random effect model. Among the 1897 original hits, thirty-two eligible RCT studies were included in the systematic review, of which seven were involved in the meta-analysis. The results showed that MBIs were effective in promoting athletes’ athletic performances (by narrative synthesis), mindfulness-level (n = 3; SMD = 0.50, 95% CI = [0.17, 0.83]; I = 45%, p = 0.16), and mindfulness-related psychological components (n = 5; SMD = 0.81, 95% CI = [0.53, 1.10], I = 77%, p =0.001), while no significant intervention effects were found on the mental health of athletes (n = 4; SMD = −0.03, 95% CI = [−0.35, 0.29], I = 89%, p < 0.001). Our findings preliminarily support the potential effectiveness of MBIs, whereas more high-quality RCTs were needed in the future.