BACKGROUND፡ Delays in seeking timely proper care pay a large number of deaths from diarrhea in children. Timely and appropriate health care seeking in under-five children with diarrhea reduces life-threatening complications. This study aimed to investigate determinants of delayed treatment-seeking for diarrheal diseases among mothers with under-five children.Method: In Debre Markos public health facilities, a facility-based case-control study was conducted among 412 mothers ((137 cases and 274 controls) from September 1 to October 15, 2020. Consecutive sampling was employed to select cases and controls. Data was collected using a semi-structured interviewer-administered questionnaire. Data were entered into Epi- Data version 4.2.1 and exported to STATA version 14 for analysis. Predictors with P-value <0.25 in the bivariable logistic regression model were candidates for multivariable logistic regression.  P-value <0.05 was used to declare statistical significance. Finally, results were presented in the form of texts and tables. RESULT: From 412 selected participants, 408 mothers (136 cases and 272 controls) were included. Female children [AOR 1.85(95% CI 1.15-2.98)], Child age < 24 months [AOR 1.64 (95% CI 1.01-2.65)], mothers’/caregivers without formal education [AOR 4.61 (95% CI 2.03-10.44)], poorest wealth index category [AOR 4.24 (95% CI 1.90-9.48)], absence of health insurance [AOR 3.04 (95% CI 1.60-5.78)], and self-medication [AOR 3.6 (95% CI 1.75-7.4)] were determinants of delayed treatment-seeking.CONCLUSION: Being female, young age, educational status of the mother, lowest wealth index category, self-medication, and absence of health insurance were determinants of delayed treatment-seeking for diarrheal diseases. Preventive care programs should target age, low socioeconomic status, and a low educational class of the motherKEYWORDS: Diarrhea; Delayed Treatment-Seeking; Children; Ethiopia