Background: We aimed to assess moderation of affective and cognitive effects of a brisk walk by urban environmental characteristics and the immediate social context. Methods: We conducted a field experiment with time (pre-walk, post-walk), type of environment (park, street), and social context (alone, with a friend) as within-subjects factors. Twenty university students reported on affective states and completed a symbol-substitution test before and after each of two 40-minute walks in each environment. The routes differed in amount of greenery, proximity to water, and presence of traffic, buildings, and other people. Results: On average, walking per se increased positive affect and reduced negative affect. Feelings of time pressure declined to a greater extent with the park walk than the street walk. Revitalisation increased during the park walks to a greater degree when alone, but it increased more during the walk along streets when with a friend. We found an inconclusive pattern of results for performance on the symbol-substitution test. Conclusions: Some psychological benefits of a brisk walk depend on the influence of the immediate social context and features of the outdoor urban environment, including natural features such as greenery and water.