Background Glycemic control is an important part of diabetes management. Strict glycemic control has been shown to reduce the long-term complications of diabetes. However, achieving good glycemic control is challenging for people with diabetes especially in resource limited settings. The aim of this study was to assess glycemic control and identify its determinants among children and adolescents with diabetes. Methods A cross-sectional study among 116 children and adolescents with diabetes was done at a pediatric endocrine clinic in southern Ethiopia. Data on socioeconomic, demographic, nutrition, and diabetes related variables were collected. Glycemic control was assessed based on glycosylated hemoglobin level. Logistic regression analysis was used to identify predictors of glycemic control. Results The mean glycated hemoglobin (HbA1c) of the participants was 9.6 ± 2.4% (81 ± 3 mmol/mol). Ninety seven (83.6%) of the study participants had poor glycemic control [HbA1c ≥7.5% (58 mmol/mol)]. The presence of lipodystrophic change at injection sites (p =0.028) and being from a family that cannot afford for insulin when there is no free supply (p =0.009) were associated with poor glycemic control. Conclusions The majority of children and adolescents with diabetes had poor glycemic control. Stakeholders shall focus on identifying strategies to improve the magnitude of poor glycemic control. More research is warranted to exhaustively list out factors contributing to poor glycemic control.