Background: This paper explores K-12 interdisciplinary learning in the humanities (IL-Humanities), an area that, until now, has seen limited research focus compared to its STEM counterparts. We asked: (1) What are the outcomes of IL-Humanities in terms of interdisciplinary competences? (2) How do learners in these environments engage in cross-disciplinary knowledge building and transdisciplinary identity work? Methods: We assessed the efficacy of IL-Humanities across ten Israeli middle schools through a pre-post intervention/comparison design, utilizing the novel Interdisciplinary Competences Assessment (ICA). Qualitative insights into the learning processes within classrooms were derived using discourse analysis methods. Findings: Students' interdisciplinary competences were found to increase following the IL-Humanities interventions. Qualitative analyses offered "thick descriptions" of the process: Students leveraged cross-disciplinary transfer of knowledge to deepen their understanding of complex phenomena and used personal narratives to engage in identity work. Contribution: This study enhances interdisciplinary education research by: (1) providing and operationalizing a model of interdisciplinary competences as an assessment tool; (2) demonstrating the effectiveness of IL-Humanities environments in developing these competences; and (3) advancing our understanding of learners' engagement with cross-disciplinary knowledge building and transdisciplinary identity work.