BACKGROUND: Evidence on beneficial associations of green space with cognitive function in older adults is very scarce and mainly limited to cross-sectional studies. OBJECTIVES: We aimed to investigate the association between long-term residential surrounding greenness and cognitive decline. METHODS: This longitudinal study was based on three waves of data from the Whitehall II cohort, providing a 10-y follow-up (1997-1999 to 2007-2009) of 6,506 participants (45-68 y old) from the United Kingdom. Residential surrounding greenness was obtained across buffers of 500 and around the participants' residential addresses at each follow-up using satellite images on greenness (Normalized Difference Vegetation Index; NDVI) from a summer month in every follow-up period. Cognitive tests assessed reasoning, short-term memory, and verbal fluency. The cognitive scores were standardized and summarized in a global cognition z-score. To quantify the impact of greenness on repeated measurements of cognition, linear mixed effect models were developed that included an interaction between age and the indicator of greenness, and controlled for covariates including individual and neighborhood indicators of socioeconomic status (SES). RESULTS: In a fully adjusted model, an interquartile range (IQR) increase in NDVI was associated with a difference in the global cognition z-score of 0.020 [95% confidence interval (CI): 0.003, 0.037; p=0.02] in the 500-m buffer and of 0.021 (95% CI: 0.003, 0.039; p=0.02) in the 1,000-m buffer over 10 y. The associations with cognitive decline over the study period were stronger among women than among men. CONCLUSIONS: Higher residential surrounding greenness was associated with slower cognitive decline over a 10-y follow-up period in the Whitehall II cohort of civil servants.