Over the past decade, mindset research has increasingly moved beyond the individual, examining how beliefs about ability are shaped and reinforced by the broader social context. This paper extends that work by focusing on peers, a powerful but underexplored influence in adolescent development. We introduce a theoretical framework for understanding peer mindset culture: the shared beliefs that peers hold and communicate about whether ability is fixed or can grow. We argue that fixed mindset peer cultures can erode students' sense of belonging by (a) triggering identity threats (e.g., stereotype threat, perceived mismatch with peers' beliefs, or social contagion of fixed norms), and (b) weakening social connection (e.g., peer selection and low support). We argue that these dynamics are especially consequential for students from minoritized backgrounds, those who are struggling academically, and those who personally endorse a growth mindset. Finally, we outline principles for designing peer-focused mindset interventions. Our goal is to provide avenues for future research and practical strategies to support all students in developing a strong sense of belonging in their learning environments.