Recent research in technology-enhanced learning environments has indicated the need to redefine the role of teachers as designers. This supports successful learners better able to adapt to twenty-first century education, in particular STEM education. However, such a repositioning of teaching as a design science challenges teachers to reconceptualize educational practice as an act of design, not in the artistic meaning of the word. Our recent research finding also indicated that teacher design knowledge (TDK) processes are often invisible to both the teacher educators and the teachers. To respond to these challenges, this paper will define TDK for STEM teachers by making TDK visible in the form of a TDK competency taxonomy. A systematic literature review was conducted to identify the characteristics of teaching practices in technology-enhanced learning environments. This TDK competency taxonomy consists of four main categories drawing on existing literature on teacher design work and teacher instructional design: data practice, design practice, knowledge creation practice, and professional teaching practice. The implications of these findings were discussed.