Introduction: Many underweight males take commercial protein supplements to increase their body weight and build muscle. Nonetheless, commercial protein supplements may cause adverse effects. This study aimed to determine the effects of resistance training exercise combined with a high protein diet on body weight and muscle mass in underweight adolescent males. Methods: A repeated measures design study was conducted on nine males aged 12-15 years with low body weight. Energy and protein requirements were calculated, and energy and protein consumptions were measured for each meal during the high protein diet without exercise (HP) period and the high protein diet with resistance exercise (HPE) period. Subjects engaged in three resistance training sessions each week during HP-E period, for eight weeks. Dietary intake, body composition, blood biochemistry, physical fitness, and self-esteem were assessed. Results: In HP-E period, resistance training exercise combined with a high protein intake (2.14 g/kg/d) increased body weight and lean tissue mass (LTM) by 0.5 kg and 0.5 kg, respectively. Resistance training during HP-E period increased arm, leg, and trunk muscle strength by 20.2%, 7.2%, and 14.5%, respectively, more than high protein diet alone during HP period. High protein intake in HP-E period did not affect blood urea nitrogen (BUN) and creatinine levels (11.0 mg/dL and 0.70 mg/dL, respectively). Conclusion: Eight weeks of resistance training combined with a high protein diet increased body weight and LTM without adverse effects. In particular, resistance exercise predominantly increased muscle strength. Kidney function was not affected by high protein consumption throughout this study.