Urban green spaces (UGS) are vital Nature-based solutions (NbS) that support biodiversity, public health, and climate resilience. Yet, their governance faces diverse challenges shaped by governance structures, sectoral dynamics, and the growing impacts of climate change. This study compares UGS governance in Korea and Germany—two countries with contrasting systems—through 30 semi-structured expert interviews analyzed using qualitative coding. Findings reveal how centralized governance in Korea enables rapid implementation aligned with global frameworks, while Germany’s decentralized model prioritizes local autonomy and EU-level policy integration, often at the cost of slower decision-making. Sectoral analysis highlights differing priorities: government actors emphasize regulatory compliance and resources, NGO focus on participation and equity, and researchers stress evidence and long-term vision. Despite structural differences, both countries share barriers such as funding gaps and policy integration challenges. These insights, contextualized through the socio-ecological systems framework, underscore the need for cross-sectoral collaboration and adaptive governance to advance UGS and NbS strategies.