Investigating the association between gestational weight gain (GWG) on low birth weight (LBW, birth weight < 2500 g) across pre-pregnancy body mass index (BMI) categories (underweight: < 18.5, normal: 18.5-24.9 and overweight/obese: ≥ 25 kg/m²) is crucial for clinical practice. While the Institute of Medicine's (IOM) 2009 GWG guidelines are widely used, evidence-based data from diverse populations is scarce, creating a global research gap. We explored how total GWG and adherence to IOM recommendations affected the odds of LBW across BMI categories in the Sri Lankan context. This nationwide prospective study evaluated 1499 maternal and singleton-newborn pairs between August 2022 and April 2024. Unadjusted and adjusted logistic regression analyses were performed. An increase in total GWG z-score was associated with decreased odds of LBW among women with underweight pre-pregnancy BMI (aOR: 0.56, 95% CI: 0.35‒0.89), but no significant association was observed among women with normal or ≥ 25 kg/m² BMI. Women with underweight BMI whose GWG was below the IOM recommended range showed higher odds of LBW than those with GWG within the recommended range (aOR 3.05, 95% CI: 1.08‒8.61). However, among women with normal or higher BMI, GWG below the recommended range was not significantly associated with LBW. These findings suggest that the association between GWG and odds of LBW varies across pre-pregnancy BMI categories. Among Sri Lankan women with underweight pre-pregnancy BMI, gaining pregnancy weight within the IOM GWG recommendations was associated with significantly lower odds of delivering an LBW newborn. This association was not observed among women with normal or higher BMI.