Urban wetlands represent critical yet increasingly threatened ecological infrastructures within rapidly expanding metropolitan regions. This chapter provides a comprehensive review of the hydrogeological and hydrogeochemical significance of urban wetlands, highlighting their roles in water purification, flood mitigation, groundwater recharge, climate regulation, biodiversity support, and carbon sequestration. Drawing on global case studies, from Africa, Asia, Europe, and the Americas, the chapter synthesizes how urban wetlands regulate water quality through nutrient cycling, redox transformations, contaminant retention, and stable isotopic interactions, while illustrating the pressures imposed by urbanization, pollution, hydrological alteration, and climate change. Special emphasis is placed on the hydrogeochemical dynamics of the Inciraltı–Çakalburnu Urban Wetland and the Gediz Delta in İzmir, Türkiye, demonstrating the interplay of seawater intrusion, geothermal inputs, agricultural return flow, and anthropogenic contamination. These insights reveal that urban wetlands are dynamic systems whose resilience depends on maintaining hydrological connectivity and chemical balance. The chapter further explores innovative management strategies and nature-based solutions, including ecological restoration, green–blue infrastructure integration, community-based stewardship, and advanced monitoring technologies, to enhance the sustainability of urban wetlands. The chapter positions urban wetlands as indispensable components of resilient and climate-adaptive cities, underscoring the need for policy frameworks that integrate hydrogeochemical understanding into urban planning and wetland conservation.