This study examined fintech adoption as a catalyst for sustainable capital formation in Nigeria from 1999-2024, with a specific focus on digital payment systems and mobile money services. The study was motivated by the growing importance of financial technology in enhancing financial inclusion, improving transaction efficiency, and mobilizing savings for investment in developing economies. An ex post facto research design was adopted, and secondary data were obtained from the Central Bank of Nigeria (CBN) Statistical Bulletin and World Bank databases covering the period 1999–2024. The study employed descriptive statistics, Augmented Dickey-Fuller (ADF) unit root test, and multiple regression analysis to analyse the data. The findings revealed that both digital payment systems and mobile money services have a positive and significant effect on sustainable capital formation in Nigeria. Specifically, mobile money services exhibited a stronger influence on capital formation compared to digital payment systems, indicating their critical role in expanding financial access among underserved populations. The regression results further showed a high explanatory power of the model, confirming that fintech adoption significantly contributes to changes in capital formation. The study concludes that fintech adoption plays a vital role in promoting sustainable capital formation in Nigeria by enhancing financial inclusion, encouraging savings mobilisation, and facilitating investment activities. It is recommended that regulatory authorities strengthen policies that support the expansion of digital payment infrastructure and deepen mobile money penetration across the country to further enhance economic development.