Basic sanitation, safe drinking water, and proper hygiene practice may lessen the burden of waterborne ill-nesses and neglected tropical diseases. The current study sought, for the first time, to evaluate household water, sanita-tion, and hygiene (WASH) practices and acute diarrhea among children under 5 years of age and their associated factors on St. Martin's Island. Two hundred one households with at least one child aged under 5 years were interviewed using a pretested semi-structured questionnaire designed according to UNICEF/WHO Joint Monitoring Program for Water Sup-ply, Sanitation, and Hygiene 2019 and on-the-spot observations. Multiple logistic regression analyses investigated the association between potential sociodemographic characteristics, WASH components, and acute diarrhea. Almost all sampled households (99.5%) had improved drinking water facilities. More than one-third (36.5%) did not have an improved sanitation facility, and open defecation was reported by 12.4% of study subjects. Only one-third of respond-ents reported washing their hands with soap or detergent after defecation, and approximately 29% of respondents reported sharing their toilet with other family members. A handwashing station with soap and water was observed in only 14.4% of cases, whereas more than 22% had none. More than one-quarter (26.4%) of children aged under 5 years were reported to experience acute diarrhea. Access to WASH facilities and the occurrence of acute diarrhea were found to be associated with a set of demographic and socioeconomic characteristics. Sanitation facilities are limited and inappropri-ate WASH practices are prevalent in the studied population with linkage to diarrhea in children; therefore, urgent attention is needed to improve WASH facilities and encourage health-promoting WASH behavior in St. Martin's Island population.