Four hundred and thirty-four children enrolled in school years 5 and 6 in the United Kingdom were administered measures of trust beliefs in peers/best friends and psychosocial functioning (internalized maladjustment, self-perceived social acceptance, social preference, and social exclusion) across an 8-month period (mean age = 9 years-9 months at Time 1). The relation between children's trust beliefs in peers or trust beliefs within best friend dyads and measures of psychosocial functioning conformed to a quadratic pattern. Compared to children in the middle range of trust beliefs, children with very low trust beliefs and those with very high trust beliefs in peers and/or within best friend dyads displayed higher internalized maladjustment, lower self-perceived social acceptance, higher social exclusion, and lower social preference. The relation between the trust beliefs and internalized maladjustment was asymmetrical, with children who held very low trust beliefs being comparatively more disadvantaged.