Protected natural areas, like national parks, are essential for conserving biodiversity and helping ecosystems adapt to climate change. This study analyzes how vegetation has changed over the last 40 years (1984–2023) in 12 national parks across mainland Spain and the Balearic Islands. Using satellite images from the Landsat program and processed in Google Earth Engine, the researchers examined how vegetation activity varied across seasons and locations. They used several vegetation indices (like NDVI, SAVI, and NDMI) to track greening and drying trends. The results show that most parks experienced an increase in vegetation, especially in high mountain areas during the growing season and Mediterranean parks throughout the year. In contrast, wetland parks like Las Tablas de Daimiel showed a worrying loss of vegetation, especially in areas that depend on water. Doñana showed mixed results: vegetation growth increased, but signs of water stress were also evident. The study also explored how topography (like slope and elevation) and natural vegetation types influenced these trends. For example, north-facing slopes and certain forest types showed stronger positive changes. The findings highlight the importance of satellite data for tracking environmental changes and supporting better conservation decisions, especially in the face of climate and human pressures.