Introduction: Birthweight is considered as a predictor of an individual’s long-term metabolic health. Gestational weight gain (GWG) is a modifiable factor that influences newborn weight. Impact of trimester-specific weight gain on birth weight is under reported. Objectives: This study aimed to assess the effect of total and trimester-specific GWG on newborn weight. Methods: A prospective study of 1537 pregnant mothers, recruited by convenient sampling technique from the antenatal clinic of Teaching Hospital Peradeniya was conducted with real time GWG assessments through pregnancy . Of them, 1060 mothers and their infants born at term without complications were selected. Multiple Linear Regression, Spearman Correlation, and Binary Logistic Regression were performed for the statistical analysis using SPSS version 27 . Results: Median GWG of the pregnant women was 9.50 (IQR- 7 .00) kg while 11.2% and 55.4% had weight gains above and below the recommendation of the Institute of Medicine, respectively . A significant positive correlation was found between newborn weight and GWG (r = 0.135, p < 0.05). Mothers with inadequate GWG had higher odds of delivering low birth weight neonates (OR-1.55, p < 0.05). Occurrence of low-birth-weight births among high body mass index mothers was always related to GWG below the recommendation. The 2nd trimester weight gain had a significant positive correlation with newborn birth weight (r = 0.098, p < 0.05). Conclusion: Inadequate GWG significantly contributes to LBW in Sri Lanka. Optimizing GWG, particularly during the second trimester, and promoting healthy body mass index prior to conception could improve the birth outcomes.