Social media use is integral to many adolescents' lives. It brings benefits but can also have detrimental effects on both physical and mental health. We conducted a systematic review examining associations between social media use, adolescent mental health (including body image, self-esteem, stress, interpersonal relationships and loneliness, anxiety, and depressive symptoms), and dietary outcomes. Quantitative studies published between 2019 and 2023 investigating both mental health and diet were searched in 11 databases. The risk of bias was appraised using ROBINS-E. Data were narratively synthesized by type of association, PROGRESS-Plus health equity characteristics, and related to social media influencers. Twenty-one studies were included, of which only one focused on influencers. Sex/gender was the only equity characteristic assessed (n = 8), with mixed results. The findings suggest significant positive correlations between social media use and both depressive and disordered eating symptoms, body dissatisfaction, and anxiety. Four studies identified body image, self-esteem, or anxiety as moderators acting between social media exposure and dietary outcomes. Policy interventions mitigating the impact of social media on adolescents-particularly body image and disordered eating-are needed, alongside follow-up studies on causal pathways, the role of influencers, equity impacts, dietary intake, and the best measurement tools to use.