Physical activity is one of the most important elements for maintaining a healthy lifestyle. The Theory of Planned Behavior successfully explains the occurrence of various behaviours. Nevertheless, it has been demonstrated that there is a need to emphasise other theoretically relevant constructs to predict physical activity. Therefore, based on the assumptions of the Theory of Self-efficacy and the Multi-Process Action Control model, the question arises whether physical self-efficacy can be a moderator and/or mediator of the relationship between the intention to engage in physical activity and the intensity of physical activity. To provide empirical data on this question, we included 188 participants aged 19 to 60 (M = 27.27 (SD = 8.21)), of whom 55.3% were women. The participants completed a physical activity self-efficacy questionnaire, a physical activity intention questionnaire, and a physical activity intensity questionnaire and answered relevant sociodemographic questions. The research results showed an indirect contribution of intention to the intensity of physical activity through physical self-efficacy (full mediation). Furthermore, the moderating role of physical self-efficacy in the relationship between intention and intensity of physical activity was determined. In more detail, the intention to engage in physical activity positively contributed to the intensity of physical activity at medium and higher levels of physical self-efficacy, while intention did not prove to be a significant predictor of physical activity intensity at a low level of physical self-efficacy. The research offers additional empirical insights into possible mechanisms underlying the manifestation of physical activity.