The development of STEAM (Science, Technology, Engineering, Liberal Arts, and Mathematics) curricula has emerged in the past decade. A pedagogical STEAM (Scaffolding, Tutoring, Engaging, Argumentation, and Modeling) model was proposed in this study to implement interdisciplinary STEAM curricula. The effects of this model on students' project competence and learning motivation were evaluated. A quasi-experimental design was adopted in the study. One hundred fourteen participants were recruited from a high school in southern Taiwan. There were 57 students in the experimental group (27 males; 30 females) and 57 in the comparison group (35 males; 22 females). The results revealed that the implementation of the pedagogical STEAM model was able to enhance the project competence and learning motivation of the students in the experimental group. Qualitative data from student interviews also revealed that the students had positive perceptions of the model. The main implication of this study is that the pedagogical STEAM model has the potential to be used as a teaching guide for the high school interdisciplinary curricula.