Nigeria's decentralized tax system presents significant challenges to efficient tax administration, with disparities across states leading to issues such as double taxation, revenue leakage, and non-compliance. The need for a harmonized tax system has become more critical as the country seeks to improve its revenue generation and create a more conducive environment for businesses. This paper examines the role of the Joint Tax Board (JTB) in creating a regulatory model that harmonizes tax collection across Nigerian states. The study outlines the complexities of the current tax structure, highlighting the fragmentation between federal and state tax authorities. It also explores the successes and limitations of the JTB in coordinating tax policies, particularly with regard to the Personal Income Tax. Drawing on international examples of successful tax harmonization, the paper proposes a hybrid regulatory model that balances state autonomy with centralized coordination. Key principles for the proposed model include the standardization of tax policies, strengthening of enforcement mechanisms, and the adoption of modern technology to streamline tax administration. Furthermore, the paper discusses the challenges of implementing a harmonized tax system, such as political resistance from states, disparities in technological capacity, and weak enforcement mechanisms. Recommendations include legislative reforms to strengthen the powers of the JTB, capacity building for state tax authorities, and increased collaboration between federal and state governments. In conclusion, this study emphasizes the pivotal role of the JTB in driving tax harmonization and highlights the need for a unified tax system to promote economic growth and improve compliance in Nigeria. Keywords: Regulatory Model, Harmonizing, Tax Collection, Nigerian States, Role, Joint Tax Board.