Evidence suggests that social identities, which provide purpose and a sense of belonging to the social world, promote resilience against psychological strain and protect well-being. This is especially important in migrant populations where adverse experiences, such as prejudice, disconnection from previous identities and issues of integration into society negatively impact well-being. Building on the social identity approach to mental health, this thesis aimed to extend knowledge on the role social identity plays on migrant mental health. In this context, we initially conducted a meta-analysis, demonstrating that increased social identification is linked with lower depressive and anxiety symptoms (Chapter 2). Given that the COVID-19 public health emergency has led to changes in people's attitudes towards minority groups, increasing prejudice and discriminatory behaviors, we conducted two studies to explore this further. Firstly, we examined whether social connectedness improves migrant resilience in adverse social situations, demonstrating that feeling socially connected is important for maintaining positive psychological well-being when facing adversities (Chapter 3). Secondly, we demonstrated that group identification shapes the majority's attitudes towards migrants, showcasing that people with authoritarian predisposition had a decreased tendency to identify with other ethnic groups, which in turn increased anti-immigrant sentiment during the pandemic (Chapter 4). Lastly, findings from our qualitative study suggest that migration and migrant status play a role in people's mental health, with social identity continuity, identity gain and embracing identity being important determinants of adjustment and positive psychological well-being (Chapter 5). The main findings of this thesis draw attention to the migrant social environment as a vital determinant of mental health, discussing implications and suggestions for future research in Chapter 6.