Increasing physical activity is an important public-health goal worldwide, but there are few published mediation analyses of physical-activity interventions in low-to-middle-income countries like South Africa undergoing a health transition involving markedly increased mortality from non-communicable diseases. This article reports secondary analyses on the mediation of a theory-of-planned-behavior-based behavioral intervention that increased self-reported physical activity in a trial with 1181 men in Eastern Cape Province, South Africa. Twenty-two matched-pairs of neighborhoods were randomly selected. Within pairs, neighborhoods were randomized to a health-promotion intervention or an attention-matched control intervention with baseline, immediate-post, and 6- and 12-month post-intervention assessments. Theory-of-planned-behavior constructs measured immediately post-intervention were tested as potential mediators of the primary outcome, self-reported physical activity averaged over the 6- and 12-month post-intervention assessments, using a product-of-coefficients approach in a generalized-estimating-equations framework. Data were collected in 2007-2010. Attitude, subjective norm, self-efficacy, and intention were significant mediators of intervention-induced increases in self-reported physical activity. The descriptive norm, not affected by the intervention, was not a mediator, but predicted increased self-reported physical activity. The results suggest that interventions targeting theory-of-planned-behavior constructs may contribute to efforts to increase physical activity to reduce the burden of non-communicable diseases among South African men.