Background Pneumonia is the one of the five-leading cause under-five morbidity and mortality in developing countries. It is the leading cause of under-five death in Ethiopia. However, data on its institutional burden and factors associated was not explored enough in the study area. Therefore, this study was aimed to asses institution-based proportion of under-five pneumonia and associated factors at Debre Tabor General Hospital.Methods Institution based cross sectional survey was conducted from January 3 to February 10/2019 on a total of 342 participants. Data was collected using structured questionnaire. Additionally, medical charts were reviewed for objective diagnosis of pneumonia. After entered by EPI info version 7 data was exported to SPSS version 20 for further analysis. The result was summarized and presented using descriptive statistics. Logistic regression was computed to assess statistical association at 95% confidence level and p-value ≤0.05.Result Among all under-five children visiting under-five outpatient department 27.5% were diagnosed for pneumonia. Participants who were not sleeping in cooking room; 0.066 (0.035,0.123)), babies who had malnutrition (AOR (95% CI); 7.897 (2.480,25.145)), no breast feeding (AOR (95% CI); 5.805 (1.477,22.821)), and mixed breast feeding (AOR (95% CI); 4.111 (1.152,14.672)) were significantly associated with increased proportion of under-five pneumonia among children visiting Debre Tabor General Hospital under-five outpatient department.Conclusion Institution-based proportion of under-five pneumonia was high in this study. Sleeping in cooking room, breast feeding and malnutrition were associated with under-five pneumonia. Therefore, health education on exclusive breastfeeding, separate cooking and sleeping room as well as improve nutrition status of children could reduce proportion of under-five pneumonia.