Fiscal and debt sustainability have become significant challenges for developing countries, including Nigeria, since the country has experienced rising debt and expanding fiscal deficits in recent decades, threatening its fiscal policies and long-term economic stability. Using data from 2000 to 2023, this study used a Vector Error Correction Model (VECM) to examine the long-term determinants of debt sustainability. The major variables studied are the government expenditure-to-GDP ratio, revenue-to-GDP ratio, debt servicing, real GDP, and institutional quality. The findings showed that, while government spending and debt servicing have a large impact on the debt-to-GDP ratio, revenue has a less direct effect. Higher government spending, when distributed correctly, can reduce the debt burden, whereas good debt servicing leads to debt reduction over time. The findings also revealed that economic growth and institutional quality have a weaker short-term impact on the debt-to-GDP ratio. The study emphasizes the gradual nature of debt adjustment, emphasizing the significance of fiscal consolidation, discipline, and strategic government expenditure. Policy recommendations include increasing spending efficiency, improving debt management measures, encouraging economic growth, building institutional frameworks, and increasing financial literacy.