This study aims to demonstrate the effect of a transtheoretical model (TTM)-based physical activity program on the exercise behavior of adolescents using WhatsApp. The study was conducted with high school students (N = 185) in a pre-/posttest, quasi-experimental design. TTM-based text messages were sent to the intervention group (n = 95) over WhatsApp every day for 8 weeks. The intervention group demonstrated more statistically significant progression in the exercising stages of change compared to the control group (n = 90) ([chi-squared] = 20.10; p = 0.00). It was also found that this group had a higher exercising self-efficacy score (t = 2.647; p = 0.009) and a higher physical activity total metabolic equivalent of task score (Z = -3.521; p = 0.000). There was no significant difference in BMI and perception of pros and cons (p > 0.05). The program was found to be effective in terms of recording progress in the exercising stages of change, increasing exercising self-efficacy, and maintaining a physical activity program.