Perceptions of the physical and social environment have been shown to be predictive of physical activity (PA) behavior. However, the mechanisms of this association have not been examined. Affective response to PA was examined as a putative mediator of the association between perceptions of the PA environment and subsequent PA behavior. As part of a PA promotion study, 59 low-active overweight or obese but otherwise healthy adults completed real-time assessments of the perceived physical and social PA environment, affective response to PA, and PA behavior over a 6-month period. As hypothesized, decreased latency to and greater duration of subsequent PA was predicted by engaging in PA with a partner (b = 17.24, SE = .45, p < .01), engaging in PA outdoors versus indoors (b = 3.70, SE = 0.67, p < .01), and perceived pleasantness of the physical (b = 0.59, SE = .17, p < .01) and social settings (b = 0.68, SE = .16, p < .01). Affective response to PA (a shift toward feeling good versus bad during PA) mediated the association between engaging in PA with a partner (a path: 0.53(.11), p < .01, b path: 0.42(.12), p < .01, ab path: 0.22(.08), 95% CI .09-.41) and perceived pleasantness of the physical (a path: .38(.02), p < .01; b path: .65(.23), p = .01; ab path: .25(.09), 95% CI .08-.43) and social setting (a path: .35(.02), p < .01; b path: .57(.23), p = .01; ab path: .20(.08), 95% CI .03-.37) and PA behavior, but not the association between engaging in PA outdoors versus indoors and PA behavior. These findings suggest that perceived environmental variables may have their effects on PA through the process of psychological hedonism.