This study examined the effect of the Treasury Single Account (TSA) on financial accountability in selected Federal Ministries, Departments and Agencies (MDAs) in the South– South region of Nigeria, with financial accountability proxied by budget execution efficiency. The study adopted a descriptive and explanatory survey research design and relied on primary data collected through structured questionnaires administered to finance-related staff of five Federal MDAs. A total of 175 valid responses were analyzed using descriptive statistics and multiple regression techniques. The results revealed that TSA implementation has a significant positive effect on financial accountability in Nigerian MDAs. Specifically, TSA coverage exerted the strongest influence on budget execution efficiency, indicating that wider integration of MDAs into the TSA framework enhances adherence to approved budgets and strengthens fiscal discipline. TSA compliance level was also found to significantly improve budget execution efficiency, underscoring the importance of enforcement and institutional discipline in achieving accountability outcomes. Furthermore, cash consolidation efficiency and revenue remittance efficiency both exhibited positive and statistically significant effects on budget execution efficiency, suggesting that efficient pooling of government cash resources and timely remittance of revenues improve liquidity management and support effective budget implementation. The regression model demonstrated strong explanatory power, confirming that TSA-related variables jointly account for a substantial proportion of variations in budget execution efficiency across the selected MDAs. The study concludes that the Treasury Single Account is an effective public financial management reform for enhancing financial accountability in Nigerian MDAs when fully implemented and complied with. It recommends full TSA coverage, stricter enforcement of compliance, strengthened cash management systems, and continuous capacity building for public finance officers to sustain accountability gains.