The convergence of financial technology and sustainability has given rise to green fintech, an innovative field leveraging cutting-edge technologies to address environmental challenges through financial solutions. This review explores the evolution of green fintech, focusing on the transformative roles of Artificial Intelligence (AI), Internet of Things (IoT), and smart contracts in developing sustainable financial services. Through a comprehensive analysis of recent literature and case studies, we examine how AI enhances ESG assessments, enables data-driven sustainable investment strategies, and facilitates green lending practices. We investigate IoT applications in environmental monitoring, supply chain transparency, and smart grid integration, highlighting their contributions to sustainable finance. The implementation of smart contracts for sustainability is explored, discussing their potential in green bonds, carbon credit trading, and renewable energy markets. The paper addresses key challenges facing green fintech, including data quality issues, privacy concerns, and regulatory uncertainties, proposing future directions for research and development. Our findings suggest that the integration of AI, IoT, and smart contracts in green fintech has significant potential to accelerate the transition to a sustainable global economy by embedding environmental considerations into financial decision-making at all levels. This article contributes to the growing body of literature on sustainable finance, providing insights for practitioners, policymakers, and researchers. It underscores the need for a multidisciplinary approach to overcome technological, regulatory, and socio-economic barriers, paving the way for a more sustainable and technologically advanced financial ecosystem.