The integration of behavioral economics and financial technology (FinTech) has provided new channels to encourage sustainable financial behavior. The rapid advancement of financial technology, or FinTech, has produced cutting-edge tools that use digital nudges and behavioral economics to promote sustainable financial practices. This study examines how customers’ sustainable financial decisions (SFD) are affected by behavioral FinTech, specifically gamification, robo-advisors, and neobanks. Using a mixed-methods approach, we employ structural equation modeling (SEM) to analyze survey data from FinTech users and sentiment analysis of user comments to improve results. According to our findings, behavioral nudges have a significant mediating role in the meaningful facilitation of SFD by gamification, robo-advisors, and neobanks. Sentiment analysis finds environmental impact, transparency, and trust among top drivers of adoption, while data privacy and algorithmic bias remain concerns. The research gives actionable recommendations for FinTech companies like incorporating live sustainability metrics and policymakers to achieve ethical nudge design and accessible access. In bridging FinTech innovation with behavioral science, this research extends the debate about scalable, sustainable finance solutions and emphasizes the imperative for longitudinal and culturally representative studies to evaluate long-term behavioral effects.