Background <p>Child undernutrition, particularly underweight prevalence among children under five, remains a major public health challenge in Sri Lanka. However, the direct impact of food price changes on underweight prevalence has received limited attention. This study investigates how changes in the prices of key nutrient-rich foods affect underweight prevalence and identifies the drivers of food price changes.</p> Method <p>A food basket of ten nutritionally relevant items was constructed to track annual food price movements by district. Panel regression models were applied to examine how district-level changes in food prices, both contemporaneous and lagged, affect underweight prevalence among children under five across 25 districts from 2017 to 2023. Determinants of the food prices were analyzed using monthly panel data from 2015 to 2023, incorporating global and domestic macroeconomic and climate-related factors.</p> Results <p>The analysis revealed that both current and lagged food prices were significantly and positively associated with underweight prevalence, with coefficients of 0.00065 and 0.00068, respectively. Food items such as rice (0.24823), dhal (0.24287), and milk powder (0.20921) were found to have the largest influence on underweight prevalence. The price determinants analysis revealed a structural break after 2020, with factors like crude oil (0.0191) and urea prices (0.0128) emerging as key drivers of domestic food prices. Domestic variables such as interest rates (-0.0305) and rainfall (-0.0089) also had significant effects. Different food prices responded differently to global and domestic factors.</p> Conclusion <p>The findings emphasize a significant positive impact of rising food prices on underweight prevalence and reinforce the importance of stabilizing prices of critical staples and protein-rich foods, while addressing both global and domestic inflationary pressures, to improve child nutrition outcomes in Sri Lanka.</p>