Background: Maternal hyperglycaemia is the central pathophysiological driver of adverse perinatal outcome in gestational diabetes mellitus (GDM), yet the comparative magnitude of risk between well-controlled and poorly controlled disease remains under-documented in South Asian tertiary-care settings. This study aimed to evaluate the association between the level of glycaemic control and perinatal outcome among women with GDM. Methods: This cross-sectional observational study included 100 women with GDM managed at a tertiary hospital in Dhaka between March and August 2017. Patients were classified as well-controlled (HbA1c <6.5, n=58) or poorly controlled (HbA1c ≥6.5, n=42) and perinatal outcomes were compared between the two groups using the chi-square test in SPSS version 20, with p<0.05 considered significant. Results: Low birth weight (<2.5 kg) and macrosomia (>4.0 kg) were each roughly twice as frequent among poorly controlled mothers (66.67%) as among well-controlled mothers (33.33%), whereas birth weight in the 3.1–3.5 kg range predominated in the well-controlled group (72.00% versus 28.00%). Postnatal maternal complications, including postpartum haemorrhage, wound infection and urinary tract infection, were markedly concentrated among poorly controlled patients (p=0.001). Neonatal morbidity, encompassing hypoglycaemia, hyperbilirubinaemia, respiratory distress syndrome, birth asphyxia and prematurity, was substantially higher among poorly controlled cases (p=0.001) and all six perinatal deaths recorded in the cohort (6%) occurred exclusively among poorly controlled patients. Conclusion: Glycaemic control status was strongly and significantly associated with neonatal morbidity, postnatal maternal complications and perinatal mortality in GDM, confirming strict glycaemic control as the principal modifiable determinant of favourable perinatal outcome.