INTRODUCTION: Previous studies indicated an association between behavior problems (internalizing, externalizing) and glycemic control (HbA1c) in youth with type 1 diabetes (T1D). The aim of this study is to examine if this association is mediated by self-confidence and mismanagement of diabetes. METHODS: Problem behavior was assessed with the Strengths and Difficulties Questionnaire. Mediating variables were assessed using the Confidence in Diabetes Self-Care-Youth and Diabetes Mismanagement Questionnaire. HbA1c was derived from hospital charts. Bootstrap mediation analysis for multiple mediation was utilized. RESULTS: 88 youths with T1D, age 11-15 y, participated. The relation between both overall problem behavior and externalizing behavior problems and HbA1c was mediated through confidence in diabetes self-care and mismanagement (a 1 b 1 + a 2 b 2 path; point estimate = 0.50 BCa CI 95% 0.25-0.85; a 1 b 1 + a 2 b 2 path; point estimate = 0.73 BCa CI 95% 0.36-1.25). CONCLUSIONS: Increased problem behavior in youth with T1D is associated with elevated HbA1c and mediated by low self-confidence and diabetes mismanagement. Screening for problem behavior and mismanagement and assisting young patients in building confidence seem indicated to optimize glycemic control.