Although there is growing consensus that population-level improvements in cardiovascular health depend upon environments that promote healthy lifestyles, evidence to support large-scale investments in neighborhood greenery to tackle obesity is inconsistent and has not focused on adults in middle-to-old age who experience the greatest burden of chronic health problems. Multilevel linear and multinomial logit regression models were fitted to investigate association between body mass index and an objective measure of green space in a sample of 246 920 Australian adults aged 45 years and older (The 45 and Up Study). Proximity to green space was constructed using catchment areas of 1 km radius around each participant. Women with over 80% proximity to green space had relative risk ratios of 0.90 (95% confidence interval: 0.83, 0.97) for overweight and 0.83 (0.74, 0.94) for obese. No similarly protective association was found for men. These results were consistent after controlling for moderate-to-vigorous physical activity (MVPA) and sitting time, each of which was favorably associated with green space proximity in men and women. Large-scale investments to enhance green spaces may promote MVPA and reduce sedentary behavior in middle-to-older-aged adults, but the impact on obesity may not benefit everyone to the same extent.