Early diagnosis and timely treatment of malaria is recognized as a fundamental element to the control of the disease. Although access to health services in Tanzania is improved, still many people seek medical care when it is too late or not at all. This study aimed to determine factors associated with delay in seeking treatment for fever among children under five in Tanzania. A three-stage cluster sampling design was used to sample households with children under five in Dodoma region, central Tanzania between October 2010 and January 2011. Information on illness and health-seeking behaviours in the previous four weeks was collected using a structured questionnaire. A multivariable logistic regression was used to investigate determinants of delay in treatment-seeking behaviour while accounting for sample design. A total of 287 under-five children with fever whose caretakers sought medical care were involved in the study. Of these, 55.4% were taken for medical care after 24 hours of onset of fever. The median time of delay in fever care seeking was two days. Children who lived with both biological parents were less likely to be delayed for medical care compared to those with either one or both of their biological parents absent from home (OR = 0.42, 95% CI: 0.24, 0.74). Children from households with two to three under-five children were more likely to be delayed for medical care compared to children from households with only one child (OR = 1.54, 95% CI: 1.04, 2.26). Also, children living in a distance ≥5 kilometres from the nearest health facility were about twice (95% CI: 1.11, 2.72) as likely to delay to be taken for medical care than those in the shorter distances. Living with non-biological parents, high number of under-fives in household, and long distance to the nearest health facility were important factors for delay in seeking healthcare. Programmes to improve education on equity in social services, family planning, and access to health facilities are required for better healthcare and development of children.