External debt sustainability includes external debt stock, external debt service and external debt to export ratio while gross domestic product is used as a proxy for economic growth. This study adopted the descriptive ex-post facto research design and the time series data on the variables were gotten from the CBN statistical bulletin (2020) and the Nigerian Bureau of Statistics (2018). The data were analyzed using the Granger Causality Test and the Ordinary Least Square regression analysis. The findings of the study revealed that external debt has positive and significant relationship with economic growth while external debt service and external debt to export ratio both has a negative relationship with economic growth. The results of the Granger Causality test revealed that unidirectional causality (effect) was found flowing from external debt to exports ratio and external debt to economic growth while there was no causality found between external debt service and economic growth in Nigeria. The study recommended that the monetary authorities should ensure that external debt incurred would ultimately result in economic growth by judiciously allocating these debts to sectors that boost output productivity and that external debt policies decisions should be founded on sustainability indicators such as external debt to export ratio, ensuring that debt is maintained below established thresholds.