The study aimed to assess the psychometric properties of a partly adapted, German version of the Weight Bias Internalization Scale (WBIS-Y) in a sample of treatment-seeking adolescents with overweight. A survey (N = 191), including measures of weight bias internalization (WBI), self-esteem, self-efficacy, health-related quality of life, and body-related locus of control, was followed by semistructured interviews (N = 10). Cronbach's α, correlation, and factor analyses were performed. Interview data were subjected to inductive content analysis. The WBIS-Y displayed good reliability and construct validity. WBI correlated negatively with self-esteem, self-efficacy, and health-related quality of life, and positively with body mass index and external body-related locus of control. Interviewees found the scale to be comprehensive, linguistically accessible, and acceptable despite its sensitive nature. The WBIS-Y is recommended for use both in research with adolescents and as a guide for mitigating the negative consequences of weight stigmatization in the treatment of pediatric obesity.