This study investigated the impact of private domestic investment, foreign direct investment, and portfolio investment on economic growth in Nigeria. The aim was to examine the interrelationship between these investment forms and GDP growth, with a focus on identifying the factors influencing their effectiveness. The study adopted an ex post facto research design and utilized time series data from 1995 to 2024, sourced from the Central Bank of Nigeria, National Bureau of Statistics, and other relevant government sources. The methodology employed includes trend analysis, descriptive statistics, and regression analysis using the Vector Autoregressive (VAR) model. Theoretical frameworks such as the Neoclassical Development Theory and the Two-Gap Theory were used to guide the analysis. The findings revealed that both private domestic investment (PDI) and foreign direct investment (FDI) have a significant positive effect on GDP growth, while portfolio investment (PIV) exhibited a negative relationship, reflecting its volatility in Nigeria’s economic context. The study concluded that while investments are vital for Nigeria’s economic development, challenges such as political instability, inadequate infrastructure, and poor governance need to be addressed to optimize investment impacts. Therefore, the study recommends enhancing governance, improving infrastructure, diversifying the economy, strengthening regulatory frameworks, and fostering technological innovation to achieve sustainable economic growth.