Background: Type 1 diabetes (T1D) requires intensive self-management, and attention deficit hyperactivity disorder (ADHD) may impair the executive functions necessary for optimal glycemic control. Emerging evidence suggests that ADHD is more prevalent among individuals with T1D, but the association with clinical outcomes, complications, and educational attainment has not been systematically synthesised in recent years. Objective: To systematically review the association between ADHD and T1D across all age groups, focusing on glycemic control, diabetes-related complications, educational outcomes, mental health, and potential moderators such as ADHD treatment status, sex, and family structure. Methods: A systematic review was conducted following PRISMA 2020 guidelines. PubMed/MEDLINE, PsycINFO, and Scopus were searched. Studies were eligible if they included individuals with T1D, assessed ADHD diagnosis or symptoms, compared with T1D without ADHD, reported original quantitative data, and were published as peer-reviewed articles. Study selection was performed using Rayyan. Risk of bias was assessed using the Newcastle Ottawa Scale (NOS). Due to heterogeneity, a narrative synthesis was performed. Results: Six studies met the inclusion criteria (two cohort, four cross sectional), encompassing over 1.48 million participants across Israel, Sweden, the United States, Norway, and Germany. Comorbid ADHD was associated with significantly higher HbA1c (mean differences +0.6% to +0.7%, p<0.01) and lower time in range (48±17% vs 59±14%, p=0.006). Adults with T1D+ADHD had higher rates of neuropathy (22.7% vs 5.8%), chronic renal failure (10.6% vs 2.5%), and limb amputation (5.3% vs 0.9%). Children with both diagnoses had 76% lower odds of finishing upper secondary school (aOR 0.24, 95% CI 0.17–0.35). Untreated ADHD was associated with worse outcomes than treated ADHD. Family structure (living with one parent and partner) increased odds of ADHD (OR 2.17, 95% CI 0.98–4.84), and sex differences favoured worse outcomes in males. Risk of bias was low in three studies and moderate in three. Conclusions: ADHD is associated with poorer glycemic control, higher complication rates, lower educational attainment, and greater depression severity in individuals with type 1 diabetes. Untreated ADHD confers the greatest risk. Systematic screening for ADHD in T1D populations, particularly those with suboptimal glycemic control or recurrent diabetic ketoacidosis, is urgently needed. Integrated, multidisciplinary care and prompt pharmacological treatment of ADHD may improve both medical and psychosocial outcomes.