This research article published by The American Society of Tropical Medicine and Hygiene, 2018Undernutrition among under-five children is a public health concern in developing countries and has been linked with poor water, sanitation, and hygiene (WASH) practices. This study aimed at assessing WASH practices and its association with nutritional status of under-five children in semi-pastoral communities of Arusha. The study was cross-sectional in design. Mother-child pairs from 310 households in four villages of Monduli and Longido were involved. Weight and height of children were measured using weighing scale and length/height board, respectively. Children's age was recorded using clinic cards. Hemoglobin level of each child was tested using Hemo Cue Hb 201 + photometer (HemoCue AB, Ängelholm, Sweden) machine. Structured questionnaire was used to gather information on WASH, child morbidity, demographic, and sociocultural characteristics. Prevalence of stunted, underweight, wasted, anemia, and diarrhea were 31.6%, 15.5%, 4.5% 61.2%, and 15.5%, respectively. Children with diarrhea 2 weeks preceding the survey ( = 0.004), children using surface water for domestic purposes ( < 0.001), and those with uneducated mothers ( = 0.001) had increased risk of being stunted and underweight. Children introduced to complementary foods before 6 months of age ( = 0.02) or belonging to polygamous families ( = 0.03) had increased risk of being stunted. Consumption of cow's milk that is not boiled ( = 0.05) or being a boy ( = 0.03) was associated with underweight. Prevalence of undernutrition among under-five children in the population under study was alarming and it could be associated with poor WASH practices and other sociocultural factors. This study underlines the importance of incorporating WASH strategies in formulation of interventions targeting on promotion of nutrition and disease prevention in pastoral communities.