Urbanization and population growth in Medan City have driven residential expansion that reduces the availability of public green open space in Medan Johor District. Within the regulatory framework of Law Number 26 of 2007, urban areas are required to provide at least 30% green open space (RTH) of the total area, consisting of 20% public RTH and 10% private RTH. This study aims to analyze the pattern and magnitude of residential expansion, its impact on public RTH availability, and the implications for environmental indicators and spatial services, focusing exclusively on the achievement of the 20% public RTH target. A quantitative approach with a descriptive-analytical method is applied through overlay analysis of 2015 and 2025 land use maps using Geographic Information Systems, complemented by demographic and spatial planning data. The findings show that residential land increased from 1,080.35 ha to 1,207.80 ha, while green open space declined from 596.08 ha to 432.15 ha, indicating a substantial reduction in public green space effectively serving the population. The proportion of total RTH decreased from 35.15% to 25.83% of the district area, which implicitly suggests that the share of public RTH falls below the mandated 20% target within the 30% policy framework. This decline has consequences for environmental quality, including reduced water infiltration capacity, higher surface temperature, and diminishing public space for social interaction, recreation, and sports. The results highlight the need to strengthen spatial regulation and land conversion control to restore and increase public RTH towards the 20% standard required by national spatial planning policy.