The rapid urbanization of the 21st century, coupled with escalating environmental challenges, necessitates the integration of green technologies within smart city frameworks to achieve sustainable urban development. This comprehensive review examines the convergence of environmental sustainability and digital innovation in creating eco-smart cities that balance technological advancement with ecological responsibility. Key green technologies analyzed include renewable energy systems, smart grids, green building technologies, intelligent transportation systems, waste management solutions, and urban agriculture initiatives. The integration of Internet of Things (IoT) sensors, artificial intelligence, and data analytics enables real-time environmental monitoring, resource optimization, and citizen engagement in sustainability practices. Case studies from leading smart cities demonstrate measurable environmental benefits including 30-50% reductions in energy consumption, 25-40% decreases in carbon emissions, and significant improvements in air quality indices. However, challenges including high initial investment costs, technological complexity, policy integration issues, and citizen behavioral change barriers must be addressed. This article analyzes current green technology implementations, quantifies environmental and economic impacts, examines integration strategies, and identifies future directions for sustainable smart city development. The findings suggest that successful green technology integration requires holistic planning approaches, stakeholder collaboration, supportive policy frameworks, and continuous innovation to create resilient, sustainable urban environments for future generations.