Background and objectives <p>Increasing urbanization and the aging global population highlight cities as vital environments for promoting healthy aging. However, the relationship between characteristics of urban green spaces and older adults' mental well-being remains unclear.</p> Research design and methods <p>This study employed participatory action research to investigate the impact of urban green space characteristics on the mental well-being of older adults in Hong Kong. Collaborating with 15 community-dwelling older adults through photovoice and design workshops, thematic analysis was used to code and analyze data.</p> Results <p>Through collaboration, four key themes were identified as key characteristics of urban green spaces and their underlying pathways to benefit late-life mental well-being: spaciousness as a psychological buffer in high-density urban living, restorative urban oases facilitating daily stress-coping strategies, inclusive design to encourage active aging in close-door urban living settings, and self-made spaces fostering co-production and ownership.</p> Discussion and implications <p>This study underscores the potential of urban green spaces for large-scale, place-based health promotion strategies to support mental well-being in healthy aging, emphasizing their roles in fostering well-being among older adults.</p>