Background: When promoting physical activity practice, it is important to consider the plausible environmental determinants that may affect this practice. We aimed to explore the impact of objectively measured Public Open Spaces (POS) on objectively measured and self-reported physical activity and the influence of weather on this association, in a Mediterranean sample of senior adults with overweight or obesity and the metabolic syndrome. Method: Cross-sectional analyses based on 218 PREDIMED-Plus trial participants aged 55 to 75 years, from Palma de Mallorca (Spain). Indicators of access to POS were assessed in a 1.0 km sausage network walkable buffer around each participant’s residence address using geographic information systems. Mean daily minutes of self-reported leisure-time brisk walking, and accelerometer objectively measured moderate-to-vigorous physical activity in bouts of at least 10 min (OM-MVPA) were measured. To investigate the association between access to POS and physical activity, generalized additive models with Gaussian link function were used. Results: Better access to POS was not statistically significantly associated with self-reported leisure-time brisk walking. A positive significant association was only observed between the distance of healthy routes contained or intersected by buffer and OM-MVPA. This association was only evident on non-rainy days. Conclusions: In this elderly population living in a Mediterranean city, only healthy routes contained or intersected by a 1 km sausage network walkable buffer influenced the accelerometer objectively measured moderate-to-vigorous physical activity in bouts of at least 10 min and rainy conditions during the accelerometer period appeared to be an important factor related to active ageing.