Background: Gestational diabetes mellitus (GDM) is defined as glucose intolerance of variable degree with onset or first recognition during pregnancy. It is the most common medical complication of pregnancy and its prevalence varies from 1-20%. GDM normally occurs in the second and third trimesters of pregnancy and is associated with several maternal and fetal complications which can cause significant morbidity and mortality. Objectives: To describe the neonatal outcomes of infants born to mothers with GDM. Method: This case-control study was conducted on a sample of 240, divided into two groups: a GDM group (n=120) and a non-GDM group (control group, n=120). Mothers and babies were recruited at the well-baby clinics in the Galle district representing six Medical Officer of Health (MOH) areas. Data were collected using medical and health records, clinic cards and an interviewer-administered questionnaire. Results: Infants of the GDM group had a higher mean birth weight (3049.50 ± 520.91g) compared to the non-GDM group (2868.13±380.73g) (p=0.002). Mean birth length was also higher in the GDM group (50.98 ± 3.12cm) than n the non-GDM group (50.20 ± 2.44cm) (p=0.031). Ponderal index and head circumference showed no significant differences. GDM group had significantly more macrosomia cases (6 vs. zero, p=0.013). Gestational age was significantly lower in the GDM group (265.61 ± 11.12 days) than in the non-GDM group (269.05 ± 10.69 days) (p=0.015). Significantly more pre-term deliveries were observed in the GDM group (17 vs. 7, p=0.031). No significant association was found between GDM and mode of delivery (X2=2.453, p=0.117). The GDM group had significantly more episiotomies (X2=19.902, p=<0.001). Neonatal jaundice (19 vs. 6, p=0.006), heart murmurs (13 vs.1, p=<0.001) and atrial septal defect (ASD) / ventricular septal defect (VSD) (10 vs 0, p=0.001) were significantly higher in the GDM group. Complication or infections in the first three months were not significantly different between the two groups (p=0.098). Conclusions: GDM was significantly associated with higher birth weights and higher birth lengths in infants, along with an increased risk of preterm deliveries. GDM was significantly linked to adverse neonatal outcomes, including macrosomia, neonatal jaundice, heart murmurs, and ASD/VSD.