Unilateral hypertrophy of the masseter muscle is a very rare pathological entity in children. Its etiology is uncertain and it requires a high degree of suspicion, as it must be differentiated from other conditions of the masseter area. As there are few pathological studies to elucidate this condition, we report a rare case of unilateral masseter muscle hypertrophy in a 16-year-old female patient with gradual onset of a painless swelling in the posterior left cheek which caused facial asymmetry with repercussions on the patient’s self-image. The diagnosis of unilateral masseter muscle hypertrophy was suggested by clinical examination, ultrasound scanning, and nuclear magnetic resonance, and was confirmed by histologic examination two years later when the patient returned for the surgical correction. The pathological findings report showed fragments of skeletal muscle with hypertrophic fibers associated with normal-sized muscle fibers in both longitudinal and transverse sections. The postoperative evaluation was favorable as both the adolescent and her family were satisfied with her look on the 14th day, 1st year, and 3rd year follow-ups. In conclusion, unilateral masseter muscle hypertrophy in adolescence is a sensitive problem due to the psychological implications of facial appearance. Definite diagnosis and treatment of the hypertrophied muscle is the ideal solution.