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Cognitive functioning is one of the most important indicators of healthy ageing. Evidence on beneficial associations of green spaces with cognitive function at older age is scarce and limited to cross-sectional studies. This study aimed to investigate the association between long-term green space exposure and cognitive decline.This longitudinal study was based on three follow-ups (10 years) of 6506 participants (45–68 years old) from the Whitehall II cohort, UK. Residential surrounding greenness was obtained across buffers of 500 and 1000 metre around the residential address at each follow-up using satellite-derived Normalised Difference Vegetation Index (NVDI) for each follow-up. A battery of four cognitive tests were applied in each follow-up to characterise reasoning, short-term memory, and verbal fluency. The cognitive scores were standardised and summarised in a ‘global cognition’ z-score. Linear mixed effects models were used that included an interaction between age and greenness to estimate the impact of greenness exposure on trajectories of cognitive decline.An interquartile range increase in NDVI was associated with a difference in the global cognition z-score of 0.020 (95% confidence interval (CI): 0.003 to 0.037, p=0.02) over 10 years. Comparing study participants of 55.7 years old, this difference was equivalent to a 4.6% slower decline over 10 years. Similar positive associations were also observed for reasoning (0.022, 95% CI: 0.007 to 0.038) and verbal fluency (0.021, 95% CI: 0.002 to 0.040), but not for short-term memory (−0.003, 95% CI: −0.029 to 0.022). We observed some suggestions for stronger associations among women and participants with secondary school education.Higher residential surrounding greenness was associated with slower cognitive decline. Further research is needed to confirm our findings and provide information on the specific characteristics of green spaces that can maximise healthy cognitive ageing.