This study examined the impact of public and private health expenditure on infant mortality in Nigeria using annual data from 1986 to 2024. Motivated by persistently high infant mortality despite rising health spending, the study employed Dynamic Ordinary Least Squares (DOLS), alongside Wald and Granger causality tests, to estimate long-run relationships and causal direction. The results show that public health expenditure significantly reduces infant mortality, with a 1% increase associated with a 0.063% decline in infant deaths. Private health expenditure also has a significant negative effect, as a 1% rise reduces infant mortality by 0.050%. Per capita GDP exerts the strongest impact, with a 1% increase reducing infant mortality by 0.157%. Granger causality results indicate that public health expenditure and per capita GDP drive reductions in infant mortality, while private health expenditure is less predictive and more reactive. The study concludes that reducing infant mortality in Nigeria depends on sustained public health investment, supportive private financing, and inclusive economic growth. It recommends increased and efficient government health spending, policies that improve access to private healthcare, and broader economic measures that enhance household welfare and living standards.