Utilizing panel data from five Northeast Asian countries spanning 2012 to 2023, this study constructs a comprehensive evaluation index system for FinTech and the digital economy. Employing nonlinear regression, mediation effect models, and spatial econometric methods, it empirically examines the impact mechanism of FinTech on regional digital economic development. The results reveal a significant inverted U-shaped nonlinear impact of FinTech on the development of the digital economy in Northeast Asia, characterized by initial promotion followed by subsequent inhibition. FinTech drives digital economic development through the dual pathways of financial inclusion and technological innovation, with these mediating channels also exhibiting inverted U-shaped characteristics. Furthermore, FinTech demonstrates significant spatial spillover effects, initially manifesting as a “siphoning effect” that inhibits the development of neighboring regions, later transforming into a “radiation effect” that fosters regional synergistic growth. The findings underscore the need for optimized FinTech development pacing, enhanced regional policy coordination, and robust cross-border regulatory frameworks