BackgroundDiarrheal disease is one of the leading causes of mortality and morbidity among under-five children in the world and often results from contaminated food and water. The aim of this study was to assess the prevalence of diarrheal disease among under-five children visiting the pediatric ward in University of Gondar Comprehensive Specialized Hospital and its association with their mothers’ handwashing knowledge.MethodsAn institutional-based cross-sectional study was conducted from May to July 2019 among under-five children visiting the pediatric ward in University of Gondar Comprehensive Specialized Hospital. An interviewer-administered questionnaire was used to collect the data. Chart review was undertaken using a data abstraction form. A simple random sampling technique was used to select the study participants. Data were entered using Epi Info version 7 and analyzed using STATA version 14.0. A binary logistic regression analysis was employed between dependent and independent variables to determine association. The statistical significance was declared at P<0.05.ResultsIn this study, the prevalence of diarrheal disease among under-five children visiting the pediatric ward in University of Gondar Comprehensive Specialized Hospital was 30.09% (95% CI: (26%, 35%)). Age of mothers (AOR=3.72, 95% CI: (1.67, 8.28)), mothers’ educational status (AOR=0.44, 95% CI: (0.23, 0.80)), malnutrition (AOR=6.72, 95% CI: (3.44, 13.11)), and maternal knowledge of handwashing (AOR=0.49, 95% CI: (0.27, 0.90)) were factors associated with diarrheal disease among under-five children.ConclusionThe prevalence of diarrheal disease was higher in the current study, which is a major public health concern. Age of mothers, malnutrition, and mothers’ handwashing knowledge and educational status were significantly associated with diarrheal disease of under-five children. In order to reduce diarrheal disease and improve child health, attention should be given to improving mothers’ educational status and knowledge regarding handwashing.