This study reports on the design of a video-based instrument to assess student teachers' parent-teacher communication competences (PTCC) in a reliable and valid way. PTCC refer to the competences needed to communicate successfully with parents during conversations such as parent-teacher conferences. Taking into account both conceptual and methodological challenges regarding the measurement of competences, several methodological steps were followed to design a video-based instrument focusing on the assessment of student teachers' perception, interpretation and decision-making skills (PID skills) regarding parent-teacher communication: designing video vignettes presenting realistic job situations regarding parent-teacher communication; developing instructions to elicit student teachers' PID skills; designing a coding scheme to analyse student teachers' responses; and implementing the instrument (n = 269) to test and refine it. This ultimately resulted in an instrument consisting of (1) three video vignettes, (2) a series of open-ended questions eliciting student teachers' PID skills, and (3) three coding schemes to develop a perception score, an interpretation score and a decision-making score for PTCC. The (4) implementation of the instrument confirmed that this innovative video-based tool might be a promising approach to assess student teachers' PTCC. To conclude, the quality and contribution of the measurement instrument and the implications for further research are discussed.