A minority of child abuse cases is recognized by professionals, making the role of non-professional bystanders essential. The stages of change construct, as proposed by the Transtheoretical Model, may provide a useful approach to explain non-professional helping behavior. The objective of this study was to test the applicability of the stages of change construct by (i) assessing whether cognitive determinants distinguish between the stages, (ii) testing the predictive value of the stages for future helping behavior and (iii) examining the mediating role of the stages in the relation between previous and future helping behavior. Data of 126 adult non-professional bystanders were analyzed. Respondents were questioned by telephone or via an Internet questionnaire, at baseline and after a follow-up of about two months later. Attitude toward helping was significantly less positive in pre-contemplation than in the other stages, and self-efficacy expectations were significantly higher in preparation compared with the other stages. Moreover, baseline preparators were more likely to conduct future helping behavior than those in the two earlier stages. Finally, the stages of change mediated the relation between previous and future helping behavior. Initial support was found for the applicability of the stages of change construct for helping behavior by non-professional bystanders.