This study investigates the extent, drivers, and consequences of land fallowing in the Eastern Gangetic Plains (EGP) region spanning India, Nepal, and Bangladesh. The region comprises a critical agricultural landscape facing rural poverty and food insecurity. Despite fertile conditions, widespread land fallowing during the kharif and rabi seasons reduces productivity and threatens household food security. Using survey data from 1440 farm households across 72 villages, probit regression models identify determinants of fallowing decisions. To estimate causal impacts on food insecurity, which is measured using the Food Insecurity Experience Scale (FIES), the study applies the Inverse Probability Weighted Regression Adjustment (IPWRA) method, which accounts for selection bias in observational data. Results show that 31% of households fallow land during the rabi season, with the highest incidence in India (41%). Key factors include limited irrigation, capital constraints, labor shortages, and climate shocks. Causal analysis reveals that fallowing raises the probability of food insecurity by 15 percentage points in India and 9 points in Nepal. The study recommends improving irrigation access, expanding direct benefit transfers, strengthening crop insurance, and tailoring extension services to reduce fallowing and enhance food security. These findings also highlight the importance of context-specific policies to address land use inefficiencies and improve rural livelihoods in South Asia.