Objectives To examine development in illness perceptions of type 1 diabetes across adolescence and relationships with intelligence, diabetes responsibility, and diabetes outcomes. Methods Illness perceptions were measured via the Illness Perceptions Questionnaire at 3 times, every 6 months in 213 adolescents (M age = 13.00; SD = 1.54) with type 1 diabetes. Intelligence and adolescents’ perceived responsibility for diabetes were examined, and adolescents’ report of adherence and quality of life (QOL), and glycosylated hemoglobin (HbA1c) from medical records addressed diabetes-related outcomes. Results Linear growth models showed significant increases in perceptions of diabetes coherence, chronicity, consequences, personal and treatment control, and decreases in diabetes cyclicality and parental control across time. More favorable illness perceptions were generally associated with adolescent intelligence at baseline, more adolescent responsibility for management, better adherence and QOL, and lower HbA1c at each time point. Conclusions Results suggest that adolescents develop complex illness perceptions, which are associated with better diabetes management.