Aim The aim of this study is to investigate the effect on health, well-being and weight loss of a motivational support programme provided for adolescents who were overweight and obese. Methods This was a randomized controlled trial. Sixty-four adolescents (39 girls and 25 boys) who were overweight and obese and consulted at our polyclinic from January 2015 to February 2016 comprised the study population. The sociodemographic characteristics, well-being levels and physical parameters of the adolescents were evaluated. Routine follow-ups were practiced for both groups. The adolescents in the experimental group had motivational interviews and were sent reminder messages over a 6-month period, and education programmes were conducted with the parents. Results After 6 months, the body mass index values had decreased in the experimental group compared with baseline, with no decrease in the controls. Scores for the Paediatric Quality of Life total scale and physical health and psychosocial health subscale points were increased in experimental and control groups, but the quality of life scale points of the experimental group were higher than the control group. In the experimental group, aspartate aminotransferase, alanine aminotransferase, insulin, homeostasis model assessment of insulin resistance (HOMA-IR) and total cholesterol were decreased. Conclusion The motivational support programme affected the adolescents' health positively and can be recommended as a routine nursing intervention.