Background Body image is considered as central to many aspects of human functioning including emotions, thoughts, behaviors and relationships. Our study aimed to investigate the prevalence of body image distortion and dissatisfaction among adolescents and to assess their main determinants. Methods This was a cross-sectional school-based study conducted among school-adolescents in the south of Tunisia, between October 2017 and February 2018. Results Among 1210 school-adolescents, body image distortion and dissatisfaction prevalence were 44.8% and 42.4%, respectively. Multivariate analysis showed that being in the 16–18 years age group [adjusted odds ratio (AOR) = 1.28, p = 0.046], low family financial situation (AOR = 1.88, p = 0.014), as well as high frequency of eating pasta (AOR = 1.3, p = 0.04) and fast-food consumption (AOR = 1.7, p = 0.042) were independently associated with under-estimated body image. Skipping breakfast (AOR = 1.9, p = 0.017) and having one obese parent (AOR = 1.9, p = 0.01) were independently associated with higher frequency of over-estimated body image. Regarding body image dissatisfaction, independent factors associated with desire to lose weight were: female gender (AOR = 1.53, p = 0.007), high income family financial situation (AOR = 2.1, p = 0.008) and having one parent who is obese (AOR = 2.21, p < 0.001). However, frequent fast-food consumption (AOR = 1.9, p = 0.038) and eating between meals (AOR = 1.57, p = 0.01) were independently associated with a higher desire to gain weight. Conclusions Our study highlighted that the prevalence of body image distortion and dissatisfaction were substantially high among adolescents. Their determinants included socio-demographic factors and lifestyle behaviors. Increased awareness among parents, educators and public health planners may help adolescents improve accuracy of body image attitudes.