Background: Globally, in 2017, there are nearly 1.7 billion cases of childhood diarrheal diseases and it is the second most important cause of morbidity and mortality among under-five children in low-income countries including Ethiopia. Sanitary conditions, Poor housing, unhygienic environment, inadequate safe water supply, cohabitation with domestic animals that may carry human pathogens, and lack of storage facilities for food combining with socio-economic and behavioral factors are the common determinates of diarrhea diseases and had a large impact on diarrhea incidence in most of the developing countries Methods: A Community-based unmatched case-control study design was conducted on 407 systematically sampled under-five children of Jimma Geneti District (135 with diarrhea and 272 without diarrhea) from May 01 to 30, 2020. Data was collected using an interview administered questionnaire and observational checklist adapted from the WHO/UNICEF core questionnaire and other related literature. Descriptive, bivariate, and multivariate binary logistic regression analysis were done by using SPSS version 20.0 Result: Socio-demographic determinants such as being a child of 12-23 months age (AOR 3.3, 95% CI 1.68-6.46) and parents/legal guardian’s history of diarrheal diseases (AOR 7.38, 95% CI 3.12-17.44) were significantly associated with diarrheal diseases among under-five children. Environmental and Behavioral factors such as unavailability of handwashing facility nearby latrine (AOR 5.22, 95% CI 3.94-26.49), lack of hand-washing practice at critical times (AOR 10.6, 95% CI 3.74-29.81), improper domestic solid waste disposal practice (AOR 2.68, 95% CI 1.39-5.18) and not vaccinated against rotavirus (AOR 2.45, 95% CI 1.25-4.81) were found important determinants of diarrheal diseases among under-five children. Conclusion: Unavailability of hand-washing facility nearby latrine, parent’s/legal guardian’s history of last two weeks diarrheal diseases, improper latrine utilization, lack of hand-washing practice at critical times, improper solid waste disposal practices, and rotavirus vaccination status were the determinants of diarrheal diseases among under-five children identified in this study. Thus, promoting households through the provision of continuous and modified health information on the importance of sanitation, personal hygiene as well as vaccination against rotavirus, which is fundamental to decrease the burden of diarrheal disease among under-five children.