BACKGROUND: Timely and appropriate health care seeking for diarrhea of under-five children is important to reduce severe and life-threatening complications. However, different findings indicate that mothers of under-five children often delay in seeking care which in turn contributes to the death of large number of children without ever reaching a health facility. Therefore, a proper pinpointing of determinants of delay in seeking care informs intervention strategies for health service planners. OBJECTIVES: Of this study was to identify the determinants of delay in care seeking for diarrheal disease among mothers/caregivers of under-five children in public health facilities of Arba Minch town, South Ethiopia, 2019. METHODS: Facility based case control study was conducted from March 4 to April 30, 2019. Total sample size was 400. Cases were selected by systematic random sampling technique while controls were mothers of under-five children with signs and symptoms of diarrhea who came to the same health facility within 24 hours following cases. Data was collected by using pretested structured questionnaire by three data collectors and entered into EpiData V4 and exported to SPSS V23 for further analysis. Bivariable logistic regression was done to identify variables candidate for Multivariable LR at p-value<0.25. Multivariable logistic regression was done and p-value <0.05 and 95%CI of AOR was used to declare statistical significance. RESULT: Female sex[AOR = 1.93, (95%CI: 1.11,3.36)], child age <24 months[AOR = 4.47,95%CI:2.51,7.97)], mothers'/caregivers without formal education[AOR = 6.90, (95%CI:3.10,15.37)], and attended primary school [AOR = 3.12,(95%CI:1.44,6.73)], poorest household wealth index category[AOR = 2.81, (95%CI:1.20,6.58) and poor household wealth index category [AOR = 2.61,(95%CI: 1.12, 6.09)], mothers/caregivers who did not visit health facility to first episode diarrhea [AOR = 4.55, (95%CI:2.41,8.59)], mothers/caregivers who were satisfied in the last six month visit [AOR = 0.29, (95%CI:0.15,0.55)], and poor perceived health care professionals respect[AOR = 4.91, (95%CI:2.64,9.15)] were important determinants of delay in seeking care. CONCLUSIONS: Sex and age of the child, educational status of the mother/caregiver, poor wealth index category, not visiting health facility at first response, satisfaction with the care and examination, and respect of health care professionals were important determinants of delay in seeking care among mothers/caregivers of under-five children with diarrhea illness. All concerned body should focus interventions on poor and less educated mothers/caregivers with emphasis on female children and <24 months. Health workers are needed to provide respectful service to promote satisfaction level of clients.