Objective: For adolescents with Type 1 diabetes, maintaining optimal daily blood glucose control is a complex self-regulatory process that likely requires self-control. This study examined whether higher self-control was associated with lower daily negative affect about diabetes and, in turn, better daily blood glucose control, that is, lower mean daily blood glucose (MBG) and smaller standard deviations of daily blood glucose (SDBG), through 2 paths: (1) self-control maintaining lower mean level of negative affect and (2) self-control buffering the association of the number of daily diabetes problems with daily negative affect. Method: Adolescents (M age = 12.87 years) with Type 1 diabetes (n = 180) completed an initial survey containing a self-report measure of self-control. Nightly electronic diaries were completed for 14 days during which adolescents reported daily problems with and negative affect about diabetes and used a study-provided blood glucose meter. Results: Hypotheses were examined through multilevel modeling. Lower mean levels of daily negative affect partially mediated the relation between higher adolescent self-control and lower MBG. Adolescent self-control also buffered the association of the number of daily problems with daily negative affect, and smaller fluctuations in daily negative affect were associated with lower SDBG. Conclusions: Adolescent self-control is associated with daily affect regulatory processes that may influence MBG. However, fluctuations in daily negative affect about diabetes may represent a unique within-subject daily process associated with SDBG. These findings suggest that studies examining daily disease processes and interventions targeting daily affect regulation may be important to improving health in adolescents with Type 1 diabetes.