When promoting physical activity (PA) participation, it is important to consider the plausible environmental determinants that may affect this practice. The impact of objectively-measured public open spaces (POS) and walk-friendly routes on objectively-measured and self-reported PA was explored alongside the influence of rainy conditions on this association, in a Mediterranean sample of overweight or obese senior adults with metabolic syndrome. Cross-sectional analyses were undertaken on 218 PREDIMED-Plus trial participants aged 55(-)75 years, from the city of Palma, in Mallorca (Spain). Indicators of access to POS and walk-friendly routes were assessed in a 1.0 and 0.5 km sausage network walkable buffers around each participant's residence using geographic information systems. Mean daily minutes of self-reported leisure-time brisk walking, and accelerometer objectively-measured moderate-to-vigorous PA in bouts of at least 10 min (OM-MVPA) were measured. To investigate the association between access to POS and walk-friendly routes with PA, generalized additive models with a Gaussian link function were used. Interaction of rainy conditions with the association between access to POS and walk-friendly routes with OM-MVPA was also examined. Better access to POS was not statistically significantly associated with self-reported leisure-time brisk walking or OM-MVPA. A positive significant association was observed only between distance of walk-friendly routes contained or intersected by buffer and OM-MVPA, and was solely evident on non-rainy days. In this elderly Mediterranean population, only access to walk-friendly routes had an influence on accelerometer-measured PA. Rainy conditions during the accelerometer wear period did appear to modify this association.