The Medical Students' Non-Technical Skills (Medi-StuNTS) behavioural marker system (BMS) is the first BMS to be developed specifically for medical students to facilitate training in non-technical skills (NTS) within immersive simulated acute care scenarios. In order to begin implementing the tool in practice, validity evidence must be sought. We aimed to assess the validity of the Medi-StuNTS system with reference to Messick's contemporary validity framework.