Objectives: This study aimed to outline water, sanitation, and hygiene (WASH) practices for children under 5 years of age among the households of St. Martin’s Island as well as examine the role of water, sanitation, and hygiene on child nutritional outcomes. Methods: A total of 256 children under 5 years of age were anthropometrically measured, and multiple linear regression was performed to understand the relationship between combined water, sanitation, and hygiene score and child nutritional outcomes: height-for-age, weight-for-age, weight-for-height z-scores. Results: The majority of the sampled households had improved drinking water facilities. Over one-third of the participants (36.5%) did not have access to better sanitation, and 12.4% reported defecating in open places. Just over one-third of those surveyed reported washing their hands with soap or detergents after defecation, with more than 22% of people not having access to a handwashing station on their premises. Stunting, wasting, and underweight were found to be prevalent in 34.4%, 17.6%, and 18.9% of the sampled children, respectively. Height-for-age (0.08 SDs) and weight-for-age (0.014 SDs) were greater on average in children from households with improved water, sanitation, and hygiene conditions, but weight-for-height did not significantly differ from those from WASH-unimproved households. Conclusion: Certain aspects of WASH, particularly, sanitation facilities and hygiene practices were not up to the mark in St. Martin’s Island, and consistent with findings from other studies, our study findings underlined the potential benefits of improved water, sanitation, and hygiene practices in addressing child undernutrition.