ImportanceResearch shows that children and adolescents with type 1 diabetes (T1D), compared with their peers without diabetes, have a greater risk of psychiatric disorders. However, no study has comprehensively examined whether having psychiatric disorders is associated with educational outcomes in children and adolescents with T1D.ObjectiveTo investigate educational outcomes in children and adolescents with T1D with and without psychiatric disorders.Design, Setting, and ParticipantsThis cohort study used data from multiple Swedish registers. The main study cohort included individuals born in Sweden between January 1, 1973, and December 31, 1997, who were followed up from birth through December 31, 2013. Data analyses were conducted from March 1 to June 30, 2022.ExposuresType 1 diabetes and psychiatric disorders (including neurodevelopmental disorders, depression, anxiety disorders, eating disorders, bipolar disorder, psychotic disorder, and substance misuse) diagnosed before 16 years of age.Main Outcomes and MeasuresAchieving educational milestones (completing compulsory school [primary and lower secondary education], being eligible to and finishing upper secondary school, and starting and finishing university) and compulsory school performances.ResultsOf 2 454 862 individuals (51.3% male), 13 294 (0.5%; 53.9% male) were diagnosed with T1D (median [IQR] age at diagnosis, 9.5 [6.0-12.5] years), among whom 1012 (7.6%) also had at least 1 psychiatric disorder. Compared with healthy individuals (without T1D and psychiatric disorders), individuals with T1D alone had slightly lower odds of achieving the examined educational milestones. However, those with both T1D and any psychiatric disorder had much lower odds of achieving milestones, including completing compulsory school (odds ratio [OR], 0.17; 95% CI, 0.13-0.21), being eligible for (OR, 0.25; 95% CI, 0.21-0.30) and finishing (OR, 0.19; 95% CI, 0.14-0.26) upper secondary school, and starting (OR, 0.36; 95% CI, 0.29-0.46) and finishing (OR, 0.30; 95% CI, 0.20-0.47) university. They also showed lower grade point averages for compulsory school subjects. These findings remained similar in sibling comparison analyses, suggesting independence from familial confounding.Conclusions and RelevanceIn this cohort study of Swedish-born children and adolescents, those with T1D alone had minor difficulties with their educational outcomes, whereas those with both T1D and psychiatric disorders had universal long-term educational underachievement. These findings highlight the importance of identifying psychiatric disorders in pediatric patients with T1D and the need for targeted educational intervention and support to minimize the education gap between the affected children and their peers.