Protected areas are essential for safeguarding biodiversity, yet their long-term environmental trajectories and stability remain insufficiently quantified at the national scale. This study evaluates two decades (2004–2023) of satellite-derived environmental dynamics within Poland’s Natura 2000 Special Areas of Conservation (SACs) using Landsat-based indicators: NDVI (vegetation greenness), TC Wetness (surface moisture), NDBSI (surface exposure related to land transformation), and LST (land surface temperature). Non-parametric trend analyses (Mann–Kendall and Sen’s slope) reveal that SACs maintained consistently high vegetation greenness with slower rates of increase than surrounding landscapes, consistent with relatively stable, long-established vegetation states. TC Wetness showed no consistent long-term directional trend, however, SACs exhibited slightly smaller declines during extreme drought periods, indicating greater persistence of moisture-related surface conditions. NDBSI declined more rapidly within SACs, suggesting lower levels of exposure surfaces commonly associated with anthropogenic land transformation, while LST remained statistically flat but consistently lower than in surrounding areas, indicating more stable thermal surface conditions. Taken together, these results indicate that areas currently designated as Natura 2000 sites follow more stable and internally consistent long-term environmental trajectories than the surrounding landscape, under shared climatic and land-use pressures. Our findings underline the importance of long-term remote sensing for assessing environmental trajectories and relative stability within protected area network.