The aim of this study was to analyse the effects of a school-based intervention to promote physical activity, utilising the postulates of the trans-contextual model of motivation. The study examined two separate classes of elementary school students (mean age 11.28?years), one of which served as the control group (n?=?26) and the other as the experimental group (n?=?21). The intervention in the experimental group consisted of showing videos related to physical activity participation, conducting discussions and doing tasks related to the content presented in the videos and conducting family discussions. Autonomy support from teachers, peers and parents; motivation in physical education and leisure time physical activity; the different variables of theory of planned behaviour; and physical activity, were measured before and after the intervention. Results showed an increase in teacher autonomy support, identified regulation in physical education, autonomy support from parents and peers, integrated and identified regulation in leisure time physical activity, control, subjective norm, intention and physical activity in the experimental group. Furthermore, this increase resulted in post-intervention differences across groups in such variables. Results are discussed in relation to the important role of families in the promotion of physical activity participation.