The existence of the tax gap challenge in the country makes it inevitable to identify the structural and institutional factors affecting tax capacity by applying various theories, including economic complexity theory. The present study investigated the effect of the economic complexity index on tax revenues in the Iranian economy by separating short-term and long-term effects during the period 1991 to 2023, using the Auto Regression with extended lags (ARDL) method. The findings show that in the short run, the Economic Complexity Index, inflation rate, and oil revenues have a negative and significant effect, while industrialization and trade openness have a positive and significant effect on tax revenues. In the long run, the Economic Complexity Index, industrialization, per capita income, and trade openness have a positive and significant effect, whereas inflation rate and oil revenues have a negative and significant effect on tax revenues. These findings are fully consistent with the Tax Modernization Theory, Kaldor's Growth Laws, the Tanzi Effect, and the Fiscal Dutch Disease Theory. Based on the findings, reducing budget dependence on oil, shortening the time lag of tax collection, designing temporary tax incentives during the transition period toward a complex economy, and investing in high-productivity industries are proposed as the most important strategies for increasing tax revenues in Iran.