Background: According to global estimates, one in seven adolescents aged 10–19 (14%) experiences mental health issues, most of which remain undiagnosed and untreated. Self-esteem is closely associated with adolescent mental health, affecting their academic performance, relationships, and emotional well-being. Objective: This study aimed to assess the self-esteem levels of middle school students using Rosenberg's Self-Esteem scale (RSES). Material and Methods: A cross-sectional study was conducted among 154 students from grades 6 to 9 in a general education school in Ulaanbaatar. The Rosenberg’s Self-Esteem scale which has a standard mean score of approximately 22, was used for assessment. Scores below ≤14 indicate low self-esteem, scores between 15–25 indicate average self-esteem, scores under ≥26 indicate high self-esteem. The Ethics committee of the University of Sociology and Psychology approved this study. Data were analyzed using Microsoft Excel and SPSS-23, applying frequencies, descriptive statistics, chi-square tests, one-sample T-tests to compare the mean score with the standard value, one-way ANOVA to examine differences between groups, and correlation analysis. Statistical significance was considered at p≤0.05. Results: The students mean self-esteem score was 17.8 (SD±4.42), significantly lower than the standard score of 22 (p<0.001), supporting our hypothesis. Statistically significant differences in self-esteem were found between grades 6, 8, and 9 (p<0.05), while no significant differences were observed by gender or age (p>0.05). Among the participants, 24.7% (n=38) had low self-esteem (score ≤14), 72.1% (n=111) had average self-esteem (scores 15–25), and only 3.2% (n=5) demonstrated high self-esteem (score ≥26). Conclusion The self-esteem of middle school students was significantly lower than the standard average. Nearly one in four students demonstrated low self-esteem, suggesting the need for early interventions to support the mental well-being of adolescents.