Psychological self-talk is commonly used to help children and adolescents perform better in sport, yet research findings have been scattered and difficult to interpret as a whole. This study set out to bring clarity by systematically reviewing and statistically combining evidence on how self-talk interventions affect both performance and psychological outcomes in young athletes. Following PRISMA guidelines, six electronic databases were searched for randomized and quasi-experimental studies involving participants aged 8–19 years. Interventions included instructional self-talk (focused on technique and task execution) and motivational self-talk (focused on effort and encouragement), delivered either alone or as part of broader training programs. In total, 28 studies involving 1,642 young athletes were included. Overall, self-talk interventions led to meaningful improvements in sport performance and psychological functioning. Performance outcomes showed a small-to-moderate improvement, while psychological outcomes such as self-efficacy, confidence, emotional regulation, and attentional control showed moderate benefits. Instructional self-talk was particularly effective for precision-based skills, whereas motivational self-talk worked better for endurance and effort-driven tasks. The benefits were stronger for adolescents than for younger children and were more pronounced in programs lasting longer than four weeks. Although study quality was generally moderate, common limitations included challenges with blinding and ensuring consistent delivery of interventions. Importantly, there was no evidence of substantial publication bias. Overall, the findings show that structured self-talk is a practical and effective psychological tool for enhancing both performance and mental skills in youth sport. Its effectiveness depends on the athlete’s age, the type of task, and how long the intervention lasts, underscoring the importance of developmentally appropriate approaches. Future research should use longer-term designs, standardized measures, and theory-based frameworks to strengthen understanding and support effective application across diverse youth sport settings.