The study examines the interactive effects of infrastructural and human capital development on economic growth in Nigeria over the period 1990 to 2023. Anchored on the New Growth Theory, the study adopts a Simultaneous Equation Model (SEM) framework and employs the Two-Stage Least Squares (2SLS) technique to account for endogeneity and feedback effects among the key variables. The results indicated that infrastructural development had a negative and statistically significant impact on growth when evaluated independently, highlighting inefficiencies, inadequate project execution, and underutilization in Nigeria's infrastructure sector. Also, human capital development showed a negative and significant effect when considered in isolation, implying that quality deficiencies, skills mismatches, and limited absorption in the labour market hinder its contribution to growth. However, the interaction term between infrastructure and human capital had a positive and significant impact on economic growth, suggesting a strong complementarity. This indicated that infrastructure and human capital enhance one another, and their joint development resulted to economic growth that neither can achieve on isolation. Among the control variables, labour force and population growth positively and significantly influenced growth, highlighting the role of demographic expansion. Inflation, government expenditure, and domestic credit to the private sector were statistically insignificant, reflecting structural and institutional inefficiencies. The study concludes that Nigeria’s growth is driven mainly by the synergy between infrastructural and human capital development. Infrastructure policy should emphasize projects that directly enhance access to education and healthcare, such as electricity for schools and hospitals, reliable transport to reduce travel barriers, and ICT networks that expand learning opportunities. Also, Transparent procurement, strong monitoring systems, and timely maintenance are essential to ensure that infrastructure spending translates into productive outcomes rather than becoming a drain on public resources.