Background: Effects of health education (HE) on physical activity (PA) are limited. Also, HE fails to address people’s personal barriers and social pressures. In contrast, “psychological inoculation” (PI) targets both topics. This research examined the effects of PI versus HE on PA-related barriers and on self-reported PA in 2 studies. Methods: Randomized controlled trials were employed. Study 1 (N = 20) took place in Britain, while study 2 (N = 40) in Belgium, with nonphysically active participants. PI exposed people to challenging sentences reflecting barriers concerning PA, which they had to refute. In study 1, PA barriers and self-reported PA levels were assessed before and a week after interventions. In study 2, the degree of refuting challenging sentences was estimated and the level of PA was assessed before and 2 months after interventions. Results: In study 1, in the PI condition alone, PA barriers significantly decreased and self-reported PA increased. Change in barriers correlated with posttreatment PA. In study 2, PA increased only in the PI group. Level of rejecting challenging sentences predicted PA later. Most group differences remained when controlling for baseline measures. Conclusions: PI is more effective than HE for increasing PA, and reducing its barriers is essential for this.