Urban sprawl and limited access to green spaces are critical challenges in post-industrial cities, affecting public health and environmental resilience. This study aims to (1) characterize brownfields in the urban core of the Górnośląsko-Zagłębiowska Metropolis (GZM) in southern Poland and (2) identify which of these sites offer the greatest potential for conversion into accessible green spaces. Using GIS analysis and a 1 km2 hexagonal grid framework, 301 brownfield sites across 13 interconnected cities were assessed based on criteria such as size, ownership, planning status, and existing structures. Population density and green space availability were analyzed in each hexagon to determine local deficits in greenery, benchmarked against WHO standards. Results show that 46 brownfields meet all transformation criteria, and greening 71 sites could significantly improve access to green space in 24 underserved zones. The study underscores the importance of strategic, data-informed brownfield re-greening to strengthen urban social and ecological resilience. The applied methodology offers a scalable planning tool that can inform sustainable redevelopment in other post-industrial regions facing similar spatial challenges.