Objective Concussion is a common ED complaint, but diagnosis is challenging as there are no validated objective measures. Use of concussion tools derived from sports medicine is common, but these tools are not well validated in ED settings. The aim of this study was to assess the ability of the Sport Concussion Assessment Tool 5th Edition (SCAT5) to identify concussion in ED patients presenting following head injury. Methods We conducted a prospective observational study of head‐injured adult patients presenting to ED between March and July 2021. ED diagnosis of concussion was used as the diagnostic standard, and we assessed the diagnostic performance of the SCAT5 test and its three subsections (Standardised Assessment of Concussion (SAC), Post‐Concussion Symptom Scale (PCSS) and Modified Balance Error Scoring System (mBESS)) against this. Results Thirty‐two head‐injured participants were enrolled, 19 of whom had a discharge diagnosis of concussion, alongside 17 controls. Median time for SCAT5 testing was 21 (interquartile range 16–27) min. Fifteen (30.6%) participants were interrupted during testing. Area under the curve (AUC) (95% confidence interval) for the SAC, PCSS and mBESS were 0.51 (0.34–0.68), 0.92 (0.84–0.99) and 0.66 (0.47–0.85), respectively. Sensitivity and specificity of sections were as follows: entire SCAT5 (100.0%, 20.0%), SAC (48.1%, 60.0%), PCSS (89.7%, 85.0%) and mBESS (83.3%, 58.8%). Using PCSS alone would have identified 17 of 19 concussions. Conclusion The SCAT5 test had a low specificity, was long and was frequently interrupted. We suggest it is not an ideal assessment to use in ED. The PCSS score performed well and was easy to complete. It may be useful as a standalone tool to simplify ED concussion identification.