Previous research on the impact of public debt on economic growth mainly highlights the nonlinearity of the relationship as well as the heterogeneity of effects among countries. Therefore, the aim of this study was to investigate nonlinearity within a sample of EU countries, with a focus on analyzing the threshold of nonlinearity in the relationship between public debt and economic growth, as well as the heterogeneity based on the level of development achieved by EU countries. Finally, the study also included testing the impact of public debt increases on higher tax burdens, as generally predicted by economic theory but not fully confirmed by existing research. The results indicate a nonlinear debt threshold of approximately 105.82% of GDP, above which public debt negatively affects economic growth, while moderate debt levels may support growth through investments in infrastructure. The heterogeneity of effects among EU countries was also confirmed, with developed countries tolerating higher debt levels, whereas less developed countries experience negative effects at lower debt levels. Lastly, the research confirms that increasing public debt leads to higher tax burdens, which can reduce private investment and consumption, thereby lowering overall economic activity.