The relationship between public debt and economic growth has garnered increasing scholarly and policy attention in recent decades. This heightened interest stems primarily from concerns regarding the potential adverse effects of elevated debt levels on economic growth, particularly in light of the sustained rise in public debt across numerous nations. A central hypothesis posits a nonlinear relationship, wherein excessive debt accumulation impedes long-term economic performance. Consequently, policymakers face a critical trade-off between the short-term stimulative effects of public debt and its potentially long-term detrimental consequences. This study investigates the debt-growth nexus within the context of Iran, employing a smooth transition regression (STR) framework to estimate threshold effects for both total government debt and government debt held by the banking system over the period 1980-2023. The empirical findings reveal threshold values of 34.14% of GDP for total government debt and 30.81% of GDP for government debt held by the banking system. Below these thresholds, increases in public debt are associated with positive impacts on economic growth. Conversely, exceeding these thresholds leads to adverse effects. Given the observed ratios of total government debt to GDP (approximately 34%) and government debt held by the banking system to GDP (approximately 30%) in 2023, the results underscore the imperative for policymakers to mitigate the potential negative consequences of exceeding these thresholds. This necessitates strategic policy interventions aimed at either preventing further debt accumulation or implementing measures to shift the threshold values upward.