Background: Generally, diarrhea is still a second major reason of death and illness in children below the age of five years. It holds breathes of around half a million under five children’s every year and causes million more to be admitted. Every year diarrhea kills around 525000 children of under five years.Although few study have been done in Tanzania to determine factors associated with diarrhea among under five children, but environment factors have not yet well addressed. Therefore, this study seeks to address environmental factors associated with diarrhoea among under-five children attending at Muhimbili National Hospital, in Tanzania. Methods: A quantitative descriptive cross sectional study design of 100 simple randomly participants were selected. Questionnaires were used to collect data form key informants who were mothers/ carers with a child of under-five suffering from diarrhoea admitted at Muhimbili National Hospital, Tanzania. Data were analysed using SPSS. Statistical tests used were mean, median, proportions and chi square. Environmental risk factors associated with diarrhoea were determined with a p value of less than 0.05. Ethical approval and permission to collect data were obtained from relevant authorities.Results: Unsuitable infrastructure for grey water disposal, improper hand washing before feeding the baby and hand washing without using clean water and soap had strongly association with the occurrence of diarrhoea in under-five children attending at Muhimbili National Hospital (MNH) with P-value of 0.00, 0.00, and 0.01 respectively.Conclusion: Childhood diarrhoea is contributed by various environmental risk factors including unimproved sanitation, lack of hand washing facilities and poor knowledge among caregivers. This calls for the importance of providing health education to the caregivers on the cause, prevention and treatment of diarrhoea among under-five children.