Introduction: Failure to Thrive (FTT) refers to poor weight gain and growth in young children and continues to be a persistent public health concern in Sri Lanka, particularly in resource-limited settings. Biological vulnerability , socioeconomic disadvantages, infections, and suboptimal feeding practices contribute to growth faltering. Objective: To identify and evaluate factors associated with FTT among children aged 6 months to 5 years attending a tertiary care hospital in the North Central Province of Sri Lanka. Method: We carried out a hospital-based case–control study over a six-month period. Fifty-five children diagnosed with FTT were enrolled along with 132 controls selected through systematic sampling. Data were collected using a pretested intervieweradministered questionnaire and anthropometric assessments. Independent associations were determined by conducting descriptive statistics, chi-square tests, and multivariable logistic regression analysis. Results: Small for gestational age (AOR=9.54; 95% CI: 3.10–29.36) was the strongest predictor of FTT. Low household income (AOR=3.40), recurrent infections within the previous six months (AOR=2.94), feeding difficulties (AOR=2.37), and delayed deworming (AOR=2.47) were independently associated with increased odds of FTT. Exclusive breastfeeding during the first six months was protective (AOR=0.39). The regression model demonstrated moderate explanatory power (Nagelkerke R²=0.47). Conclusion and Recommendation: FTT results from interconnected biological, socioeconomic, infection-related, and feeding determinants. Strengthening early identification of at-risk infants, promoting exclusive breastfeeding, improving infection prevention, and supporting vulnerable families are essential. Integrated, context-specific interventions are recommended to reduce growth faltering and improve child health outcomes.