Teacher design work has gained increasing attention by re-conceptualizing teachers as designers rather than curriculum deliverers. However, assessing teacher design work can be challenging given that there are very few research tools to assess teacher design knowledge (TDK) competencies. To fill that gap, this study proposes a survey that assesses TDK competencies in the era of digitally-mediated learning. The validity and reliability studies of the scale were carried out with 66 teachers. After the EFA, the TDK survey included 43 items from 77 items and had three factors. These factors were "data literacies practice," "design practice," and "distributed epistemic practice." Despite the limitations of the small sample size, the findings revealed that the TDK scale was a valid and reliable instrument for measuring TDK competencies. The implications of these findings were discussed.