Taxation has become a crucial source of sustainable government revenue in modern economies. One of the major challenges facing Iran's tax system is tax evasion, which is influenced by factors such as tax law complexity, lack of fairness and equity in the tax system, and high inflation rates. Tax system complexity creates confusion for taxpayers and increases compliance costs. This study evaluates the impact of tax fairness and complexity on economic growth across Iran's provinces from 2011 to 2021 using the Fully Modified Least Squares (FMOLS) method. The results indicate that tax fairness has a significant positive effect on economic growth: a 1% increase in tax fairness leads to a 0.46% increase in economic growth across all provinces. However, composite measures of tax complexity show a significant negative relationship with economic growth - a 1% increase in tax complexity results in a 0.31% decrease in economic growth across all provinces. The study also finds that inflation has a negative impact on economic growth, while government expenditures and exports show significant positive relationships with economic growth. Therefore, policymakers should prioritize simplifying tax laws, reducing tax collection costs, establishing a comprehensive tax system, and reforming the tax regime alongside implementing fiscal and monetary discipline to enhance economic growth.