Background: Shame, as a self-conscious, complex, and universal emotion, plays an important role in mental health. In adolescents, given their greater vulnerability to the development of psychological difficulties, the assessment of shame, in its various dimensions, is especially relevant. Objective: To adapt and validate the External and Internal Shame Scale for adolescents (EISS-A). Specifically, the EISS-A factor structure, reliability, and validity were analysed. Methods: The studies were conducted in two Portuguese independent samples aged 12 to 18 years old (N = 497; 58.2% female). Participants voluntarily completed a set of self-report questionnaires in the classroom. Forty-one adolescents completed the EISS-A 4-weeks after the first administration. Results: The second-order confirmatory factor analysis confirmed the EISS-A model of two related factors (external and internal shame). This model was replicated in a second independent sample. The EISS-A showed measure invariance across sex. It revealed adequate internal consistency ([alpha] = 0.85), test--retest reliability (r = 0.81), and concurrent validity with different measures of positive and negative affect. Girls showed higher shame (internal and external) scores. Age and education showed a weak positive association with shame. The EISS-A presented significant positive correlations with indicators of psychopathology (symptoms of anxiety and depression), with self-criticism and external shame (assessed by another instrument). Inversely, it showed negative associations with self-reassurance and with favourable social comparison. Conclusions: The EISS-A is a short, reliable, and useful measure for research, educational and clinical contexts with school-aged adolescents. Measuring shame may contribute to a broader understanding of its role during the most formative years of an individual's life.