Background: Social media has become an integral component of modern communication and daily lifestyle. Although social networking platforms provide educational, professional, and social benefits, excessive and compulsive usage has increasingly been associated with adverse psychological outcomes among young adults. Concerns regarding depression, anxiety, stress, sleep disturbances, loneliness, and reduced self-esteem have raised significant public health interest. Objective: To systematically review and meta-analyze the association between social media addiction and mental health outcomes among adults aged 18 years and above. Methods: This systematic review and meta-analysis was conducted according to PRISMA 2020 guidelines. Electronic databases including PubMed, Scopus, Embase, PsycINFO, and Web of Science were searched for studies published between January 2010 and April 2026. Observational studies, cohort studies, and randomized controlled trials evaluating social media addiction and psychological outcomes were included. Two independent reviewers screened studies, extracted data, and assessed methodological quality using the Newcastle–Ottawa Scale and Cochrane Risk of Bias Tool. Results: A total of 52 studies involving approximately 1.8 million participants were included. Heavy social media use was significantly associated with depression, anxiety, stress, loneliness, and poor sleep quality. Meta-analysis demonstrated a pooled odds ratio of 1.82 for depressive symptoms among individuals with problematic social media use. Increased screen exposure and compulsive engagement were strongly associated with psychological distress and reduced emotional well-being. Conclusion: Social media addiction is significantly associated with adverse mental health outcomes among young adults. Strategies promoting digital literacy, healthy online behavior, and psychological awareness are essential to minimize negative psychological consequences.