Basic sanitation, safe drinking water, and proper hygiene practice may lessen the burden of waterborne illnesses and neglected tropical diseases. The current study sought, for the first time, to evaluate household water, sanitation, 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 Supply, 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 respondents 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 inappropriate 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.