Budgetary discipline is one of the pillars of fiscal governance and a necessary condition for government efficiency and economic stability. This study aims to evaluate the level of budgetary discipline in Iran during the Sixth Development Plan (2017–2023) using an analytical–descriptive approach based on secondary data extracted from the Budget Execution Reportof the Supreme Audit Court of Iran. Drawing on the theoretical foundations of public finance and international standards such as the Public Expenditure and Financial Accountability (PEFA) framework and the IMF Fiscal Transparency Code, a Composite Budget Discipline Index (C-BDI) was developed, covering four components: “revenue realization,” “expenditure realization,” “appropriation reallocations,” and “compliance with the provisions of the annual budget act.” Data were extracted through systematic content analysis of the audit reports and aggregated using simple normalization and weighting methods. The results indicate that, although revenue and expenditure realization were relatively satisfactory in most years of the Sixth Development Plan, the high volume of appropriation reallocations beyond legal ceilings and incomplete implementation of budgetary provisions weakened budgetary discipline. Furthermore, economic shocks, the volatility of oil revenues, and weak institutional oversight mechanisms were identified as the main drivers of budgetary indiscipline. By providing a native framework for quantifying and monitoring budgetary discipline based on official data, this study can serve as a basis for designing institutional and policy reforms in Iran’s public finance system.