This paper examines the relationship between fiscal policy instruments—government expenditure, taxation, and public debt—and their effects on investment and trade dynamics in Nigeria. Fiscal policy remains a central tool for macroeconomic management in developing economies, yet Nigeria continues to face persistent challenges of low investment, weak trade performance, and escalating public debt. The study employs an ex-post facto research design and utilizes secondary time series data sourced from the Central Bank of Nigeria and the World Bank. The empirical analysis employs descriptive statistics, Augmented Dickey-Fuller unit root tests, Johansen cointegration tests, and Ordinary Least Squares (OLS) regression techniques. The findings reveal that government expenditure has a positive and statistically significant impact on both investment and trade, supporting the Keynesian view that public spending stimulates economic activity by improving infrastructure and creating an enabling business environment. In contrast, taxation exerts a negative and significant effect on both investment and trade, indicating that high tax burdens reduce profitability and weaken competitiveness. Public debt also exhibits a negative and significant impact on investment and trade, confirming the crowding-out effect whereby excessive government borrowing diverts resources from productive investment, raises interest rates, and creates macroeconomic instability. The study concludes that fiscal policy significantly influences investment and trade in Nigeria, but effectiveness depends on expenditure composition, tax structure, debt sustainability, and policy coordination. Recommendations include prioritizing capital expenditure, broadening the tax base, reducing corporate tax rates, exercising fiscal discipline, limiting domestic borrowing, and enhancing coordination among fiscal, monetary, and trade policies to promote sustainable economic development.

Keywords: Fiscal Policy, Government Expenditure, Taxation, Public Debt, Investment, Trade Dynamics, Nigeria, Economic Growth.