Prior interventions have shown limited efficacy in increasing the number of adolescents engaging in adequate physical activity (PA). Preliminary evidence suggests acceptance-based behavioral treatments (ABTs) may increase PA; however, this approach has not been tested in adolescents. This was a nonrandomized experimental pilot study that examined feasibility, acceptability, and treatment outcomes of a school-based, acceptance-based behavioral intervention for PA. Adolescents (n = 20) with low activity received a PA tracking device and were allocated to device use only or device use plus 10-weeks of ABT. PA, cardiovascular fitness, and physiological outcomes were measured pre- and postintervention. The intervention was found to be feasible and acceptable. PA, cardiovascular fitness, and physiological outcomes improved over time in the intervention group, but not in the comparison condition. This study demonstrated feasibility, acceptability, and preliminary treatment efficacy based on effect sizes for an acceptance-based behavioral intervention to increase PA in adolescents.