The aim of this study was to examine the effects of Epstein's TARGET strategies on adolescents' intentions to be physically active and leisure-time physical activity (LTPA) levels. A total of 447 secondary education students (193 females and 254 males), range age 12–17 years, were divided in two groups: control (N = 224) and experimental (N = 223). Epstein's TARGET strategies were applied by especially trained teachers only to the experimental group in their physical education (PE) classes during 12 consecutive weeks. Participants' intentions to be physically active and their LTPA levels were assessed prior to the intervention (pre), at the end of it (post-1) and 3 months after the intervention (post-2). Significant increases were observed only in the experimental group in post-1 and post-2 on both variables. PE interventions based on TARGET strategies seem to be effective increasing adolescents' intentions to be physically active, as well as time spent in LTPA. As most adolescents participate in PE, these interventions could lead to substantial public health benefits.