Background: Urban design features, such as walkable neighbourhoods and greenspace, might protect human health by enabling healthy behaviours, including increased physical activity. By definition, neighbourhoods that are ‘walkable’ are high density, whereas green spaces (e.g., parks) are inherently low density environments. We aimed to assess the impact of greenspace cover on associations of neighbourhood walkability and physical activity. Methods: We created a novel ‘green walkability’ score that integrated greenspace cover (within a 1000m walkable network buffer) with standard walkability metrics – population density, street junction density and destination density. We applied the green ‘walkability score’ to the residential addresses of UK Biobank participants living in Greater London (n= 58,587). We used logistic regression models to assess associations of ‘green walkability’ versus standard walkability scores with physical activity outcomes, adjusted for age, sex, household income and neighbourhood deprivation. Results: Higher ‘green walkability’ was beneficially associated with active commuting behaviour, active non-commute transport, International Physical Activity Questionnaire (IPAQ) responses, and achieving weekly UK physical activity guideline recommendations. Associations of ‘green walkability’ with physical activity outcomes, however, were attenuated compared to standard walkability score associations. For example, difference in odds of walking for non-commute transport between the least walkable and most walkable quintile in UK Biobank were smaller for ‘green walkability’ (odds ratio (OR) 4.46; 95% CI 4.12, 4.84) than standard walkability (OR 6.37; 95% CI 5.85, 6.94). Conclusion: Our ‘green walkability’ approach highlights the need to account for environmental correlates of physical activity (e.g., density of features) in greenspace and physical activity assessments, and might partially explain inconsistencies in direction and size of effect in the greenspace-physical activity literature. Prioritising neighbourhood density and greenspace exposure, which are inversely related in space, to support active, healthy lifestyles, is an important urban design challenge.