In countries with high child mortality rates, such as Nigeria, early intervention for common childhood illnesses (e.g., pneumonia and malaria) is essential for improving clinical outcomes. The timely reporting and treatment of fever is therefore critical in making a differential diagnosis and choosing an appropriate course of treatment. The present study aimed to investigate the prevalence and major risk factors associated with delays in seeking treatment for fever in children under five years of age in Nigeria. This study used a total weighted sample of 7,466 children under five years of age from the 2018 National Nigerian Demographic and Health Survey. Multivariable binary logistic regression modelling was used to investigate the association between predisposing, enabling, need, health service and community level factors, and delay in treatment-seeking for fever. We report the delays in seeking treatment for childhood fever that was reported by mothers in the last two weeks prior to the national survey. The prevalence for delayed treatment was 62.1% (95% confidence interval [CI]: 60.1%, 64.1%). Our findings showed that there were fewer delays in seeking treatment in children aged 24-59 months (adjusted odds ratio [aOR] = 0.79, 95% CI: 0.68, 0.93), among mothers who were formally employed (aOR = 0.84; 95% CI: 0.73, 0.96), regularly attended antenatal services (aOR = 0.76, 95%CI: 0.66, 0.88), and for those who resided in wealthier households (aOR = 0.71; 95% CI: 0.56, 0.89). Children whose mothers resided in the North-West geopolitical zone of Nigeria were less likely to delay seeking treatment for fever (aOR = 0.55; 95% CI: 0.42, 0.73). However, mothers who had an unwanted pregnancy had a higher odds of delaying treatment for childhood fever (aOR = 1.58; 95% CI: 1.05, 2.39). There were significant delays in seeking treatment for childhood fever in poorer homes found in geopolitically unstable zones of Nigeria. Mothers who were poor, unemployed, and with younger children (<12 months) often delayed seeking treatment for their febrile child. Future health promotion strategies and microenterprise schemes should target both rural and urban mothers residing in poor households. Children under 12 months of age should be a priority.