<p>This study examined the prospective association between frequency of social media use and mental health variables in adolescents. We also evaluated the impact of a school mobile phone ban on mental health outcomes among South Australian adolescents and whether social media use moderated these changes. A 2 (phone ban: yes/no) × 2 (time: baseline, follow-up) repeated-measures design was employed across five schools. Students completed electronic surveys at baseline (end of Term 2, 2023) and follow-up (early Term 3, 2023). A total of 1319 students (<i>M</i>^age = 14.50, <i>SD</i> = 1.89, 50% female) completed the baseline survey, with 603 (45.7%) completing follow-up. At follow-up, adolescents who reported higher frequency of social media posting had significantly higher psychological distress (Cohen’s <i>d</i> = − 0.29; 95% <i>CI</i>: − 0.44, − 0.14) and greater frequency of self-harm thoughts (<i>d</i> = − 0.47; 95% <i>CI</i>: − 0.62, − 0.32) and behaviours (<i>d</i> = − 0.60; 95% <i>CI</i>: − 0.75, − 0.45) compared to weekly or less frequent social media posters. No significant differences in mental health outcomes were found between ban versus no-ban groups based on social media posting levels. Our findings contribute to growing literature linking frequent social media use with poor adolescent mental health. While the school mobile phone ban showed no impact on mental health outcomes, results reinforce the need for preventive measures targeting problematic social media behaviours, including psychoeducation, parental guidance and policy measures. Future research should examine the addictive components of adolescent social media use to inform prevention and early intervention strategies.</p>