The purpose of this study was to develop a scientific foundation for a tool for assessing emotional skills in early adolescence for use by music therapists. The study included three sub-studies. In the first sub-study, a scoping review and conceptual analysis were used to define early adolescents’ emotional skills and how they are described in the research literature. The second sub-study used focus group interviews and deductive-inductive content analysis to explore how music therapists work to promote emotional skills in early adolescence and what methods they use in their work. Finally, the third sub-study identified items that could be used to assess emotional skills in early adolescence. The content validity of the developed item pool was assessed using psychometric measures. Based on the study’s results, early adolescents’ emotional skills are divided into six components: 1) expressing emotions, 2) monitoring emotions, 3) identifying emotions, 4) understanding emotions, 5) regulating emotions, and 6) the ability to use emotional knowledge at the individual level to develop and establish positive self-management and the ability to use emotional knowledge at the social level to develop and establish positive relationships. The work of music therapists to promote emotional skills was seen as multi-method and the process of developing emotional skills as a holistic one, encompassing the mental, physical and cognitive levels. The content validity of the item pool developed proved to be of high quality and can be used for further research. The study provides new insights into emotional skills in early adolescence and music therapy work on emotional skills, and an item pool that could be used to assess emotional skills development in music therapy. The study results offer a solid basis for further developing a music therapy tool for assessing emotional skills.