Background: Evidence has shown that the prevalence of body dissatisfaction in youth has been increasing in recent years and it seems that adolescence is the main culprit. Objective: This study assessed body dissatisfaction in adolescents over one year. Methods: A total of 358 adolescents of both genders aged 11 to 14 years were investigated on three occasions (T1, T2, and T3). The investigation included collecting their height and weight for calculating body mass index (BMI) and administrating the Body Shape Questionnaire (BSQ). Results: The prevalences of body dissatisfaction in males and females differed significantly. Over time, dissatisfaction increased gradually in females and decreased in males (p < 0.05): females were more dissatisfied on T3 than on T1 and T2 (F = 9.71; p = 0.001) while males were more dissatisfied on T1 than in T2 and T3 (F = 12.55; p = 0.001). The scores between occasions T1, T2, and T3 also differed. Discussion: Over a period of one year, body dissatisfaction increased in females and decreased in males. Health-promoting programs for school children should be implemented during the school year.