Background: Qualitative studies in type 1 diabetes indicate that visibility of diabetes supplies, self-care, and hypoglycemia symptoms are associated with stigma and suboptimal management. This may be particularly salient in youth who face concurrent challenges such as establishing autonomy and making vocational choices. Objective: The aim of the study was to estimate stigma prevalence in youth (aged 14-24 years) with type 1 diabetes and its associations with glycemic control. Methods: Participants, recruited largely through social media, were asked to complete a Web-based survey and to send via mail capillary blood samples for glycated hemoglobin (HbA1c) measurement. The primary definition of stigma required endorsement of one or more of 3 stigma-specific items of the Barriers to Diabetes Adherence questionnaire. These addressed avoidance of diabetes management with friends present, difficulty telling others about diabetes diagnosis, and embarrassment in performing diabetes care with others present. Poor glycemic control was defined as HbA1c>9% (ie, >75 mmol/mol; measured value when available, else self-report) and/or ≥1 severe hypoglycemic episode in the previous year (reported requiring assistance from someone else during the episode). Stigma prevalence was computed (95% CI), and associations with glycemic control were evaluated (multivariate logistic regression models). Results: Among the 380 respondents, stigma prevalence was 65.5% (95% CI 60.7-70.3). Stigma was associated with a 2-fold higher odds of poor glycemic control overall (odds ratio [OR] 2.25, 95% CI 1.33-3.80; adjusted for age, sex, and type of treatment). There were specific associations with both HbA1c>9% (75 mmol/mol; OR 3.05, 95% CI 1.36-6.86) and severe hypoglycemia in the previous year (OR 1.86, 95% CI 1.05-3.31). Conclusions: There is a high prevalence of stigma in youth with type 1 diabetes that is associated with both elevated HbA1c levels and severe hypoglycemia. Targeted strategies to address stigma are needed. Trial Registration: ClinicalTrials.gov NCT02796248; http://clinicaltrials.gov/ct2/show/NCT02796248 (Archived by WebCite at http://www.webcitation.org/6yisxeV0B) [J Med Internet Res 2018;20(4):e151]