STEM education and research has gained popularity internationally over the last decade. However, there is a lack in specifications in existing K-12 STEM classroom observation protocols of how features of an integrated STEM experience/lesson would lead to desired outcomes and how those outcomes should be measured. To bridge this gap, we propose the development of a new integrated STEM classroom observation protocol (iSTEM protocol). This article describes the ongoing development work of the iSTEM protocol, which features two creative attempts. Firstly, the productive disciplinary engagement framework is adapted to design a classroom observation protocol that provides a coherent frame of design principles to be met to achieve desired 3-dimensional pedagogical outcomes. Secondly, "interdisciplinarity" of student engagement was interpreted in terms of the extent to which students take a systematic and disciplinary-based approach to make and justify decisions during STEM problem-solving. The iSTEM protocol comprises 15 items (4-point scale) rated holistically for the extents to which evidence was found in the observed lesson for (1) the 3-dimensional pedagogical outcomes of productive interdisciplinary engagement (five items) and (2) problematising, resources, authority, and accountability design principles (10 items). The accompanying iSTEM profile visually represents and communicates the strengths and inadequacies in design principles, thus providing explanations for extents of students' productive interdisciplinary engagement. The iSTEM protocol will contribute as a research tool for STEM education researchers and as a pedagogical guide for STEM classroom teachers to improve their design of STEM learning experiences.