Rapid urbanization in the Philippines has significantly reduced the accessibility and functional quality of public green spaces, particularly in densely populated cities such as San Pedro, Laguna. Although sustainable city initiatives promote green urban development, limited attention has been given to how residents behaviorally adapt when green infrastructure is inadequate, poorly maintained, or weakly integrated into everyday urban life. This study, titled “Adaptation to the Absence of Urban Green Spaces: A Behavioral Study of Residents in San Pedro City, Laguna,” examines how urban Filipinos respond to these spatial constraints through the lens of Environmental Possibilism. Using a qualitative-dominant mixed-methods approach, the research evaluates residents’ perceptions alongside documented on-site behavioral observations. Findings reveal a strong perception of inadequacy and limited functionality among existing parks and plazas. In response, residents adapt by utilizing alternative outdoor spaces for leisure, recreation, and social interaction. Frequently used areas include sidewalks, vacant lots, basketball courts, mall open areas, and private home gardens. Although informal and not originally intended as primary green environments, these spaces effectively function as substitutes for formal public parks. The study emphasizes the need for people-centered, behavior-informed urban planning that prioritizes accessibility, integration, safety, and everyday usability to create inclusive and meaningful green environments in dense Philippine cities.