Introduction: Neonates commonly experience physiological weight loss in the first few days of life. A weight loss >10% of birth weight is considered excessive and is associated with increased risks of morbidity and mortality. Objectives: To evaluate the factors influencing weight loss among exclusively breastfed newborns during their first week of life. Method: A case-control study was conducted at the Colombo North Teaching Hospital, Ragama, in 2025. The study enrolled exclusively breastfed newborns aged 2-7 days, born at or after 34 weeks of gestation with a birth weight of at least 1500g. Cases were defined as newborns with a weight loss >10%, while controls were those with a weight loss <7%. Data were collected through maternal interviews and medical record review. Results: The study included 65 cases and 65 controls. The mean weight loss was 10.5±0.5% for cases and 4.8±1.0% for controls. A significantly higher proportion of mothers in the case group were primiparous (83.1% vs. 67.1%, p<0.05), had fewer than 6 antenatal clinic visits (63.1% vs. 21.5%, p<0.001), lacked antenatal breastfeeding counselling (69.2% vs. 38.5%, p<0.01), and delivered via caesarean section (49.2% vs. 32.3%, p<0.05) compared to control group. Other factors that were significantly associated with excessive newborn weight loss included prematurity (<37 weeks) (p<0.05), maternal cracked nipples (p<0.01), and maternal breast engorgement (p<0.001). Furthermore, newborns with excessive weight loss had a significantly higher incidence of jaundice requiring phototherapy, dehydration fever, and hypoglycaemia. Conclusions: Primiparous mothers, mothers with less than 6 antenatal clinic visits, mothers who lacked antenatal breastfeeding counselling, mothers who delivered via caesarean section, prematurity, maternal cracked nipples and maternal breast engorgement were significantly associated with excessive newborn weight loss.