Social media use has reached almost every sport context, yet its psychological implications for athletes remain inconsistently defined. Existing research suggests potential links between social media engagement and mental health outcomes, but findings vary across athlete populations, platforms, and usage patterns. The aim of this systematic review was to synthesize empirical evidence examining the relationship between social media use and mental health outcomes in athletes, and to identify key moderators, gaps, and implications for sport psychology practice. A systematic search of EBSCOhost, PubMed, Scopus, Web of Science, and Google Scholar was conducted between October and December 2025 to identify peer-reviewed empirical studies examining social media use and mental health outcomes in athlete populations. Studies were screened according to predefined eligibility criteria, and eleven studies met the inclusion criteria and were included in the final synthesis. Data were extracted and synthesized narratively due to heterogeneity in study design, athlete samples, social media measures, and psychological outcomes. Across the included studies, social media use was frequently associated with negative mental health outcomes, including anxiety, depressive symptoms, emotional distress, body image concerns, and burnout. Passive engagement, appearance-based comparison, and problematic use patterns were more consistently linked to poorer psychological outcomes, whereas active engagement and context-dependent use showed mixed or potentially protective associations. However, given the predominance of cross-sectional designs, heterogeneity in measurement, and the still limited number of studies, these findings should be interpreted cautiously. Overall, current evidence suggests that social media use may relate to athletes' mental health in complex and context-dependent ways. The strength and direction of associations appear to vary according to usage patterns, developmental stage, and competitive level. Further longitudinal, platform-specific, and elite-focused research is required to clarify mechanisms, temporal relationships, and population-specific effects.