Background <p>This study investigates the moderating role of ethnic identity in the relationship between self-esteem, peer relationships, and mental well-being among adolescent and young adult.</p> Methods <p>Data were collected from 1,336 participants (ages 14–22; 51.2% female, 48.2% male) at five secondary schools and five universities. Ethnicity was self-reported as follows: European Asian (Russians and other Slavic groups long established in Kazakhstan) – 32.1%; European (recent migrants from Europe) – 10.5%; East Asian (Kazakh, Uzbek, and Uyghur) – 10.5%; and Other (smaller or mixed ethnic groups) – 2.7%. Standardized measures included the Warwick–Edinburgh Mental Well-Being Scale (WEMWBS), the Revised Multigroup Ethnic Identity Measure (MEIM), the Perceived Level of Peer Pressure Scale (adapted from Santor et al., 2000), and the Rosenberg Self-Esteem Scale (RSES). Structural equation modelling was conducted using WarpPLS statistical software.</p> Results <p>The results revealed a direct positive association between ethnic identity and mental well-being (β = 0.35, <i>p</i> &lt; .001). Peer influence (β = 0.25, <i>p</i> &lt; .001) and self-esteem (β = 0.60, <i>p</i> &lt; .001) also significantly predicted ethnic identity. Ethnic identity significantly moderated the relationship between peer influence and mental well-being (β = 0.22, <i>p</i> &lt; .001), but its moderating effect on the relationship between self-esteem and mental well-being was not significant (β = 0.03, <i>p</i> = 0.30). Overall, these findings highlight a strong direct effect of ethnic identity on mental well-being, while demonstrating that its moderating role is specific to peer influence rather than self-esteem.</p> Conclusion <p>The study concludes that ethnic identity serves a buffering role in the mental well-being of young individuals, particularly in the context of peer relationships. Implications and future directions for research are discussed.</p>