Abstract Background The Child Sport Concussion Assessment Tool 5th Edition (Child SCAT5) was developed to evaluate children between 5 and 12 years of age for a suspected concussion. However, limited empirical evidence exists demonstrating the value of the Child SCAT5 for acute concussion assessment. Therefore, the purpose of our study was to examine differences and assess the diagnostic properties of Child SCAT5 scores among concussed and non-concussed middle school children on the same day as a suspected concussion. Methods Our participants included 34 concussed (21 boys, 13 girls; age = 12.8 ± 0.86 years) and 44 non-concussed (31 boys, 13 girls; age = 12.4 ± 0.76 years) middle school children who were administered the Child SCAT5 upon suspicion of a concussion. Child SCAT5 scores were calculated from the symptom evaluation (total symptoms, total severity), child version of the Standardized Assessment of Concussion (SAC-C), and modified Balance Error Scoring System (mBESS). The Child SCAT5 scores were compared between the concussed and non-concussed groups. Non-parametric effect sizes ($r=\frac{Z}{\sqrt{n}}$ r = Z n ) were calculated to assess the magnitude of difference for each comparison. The diagnostic properties (sensitivity, specificity, diagnostic accuracy, predictive values, likelihood ratios, and diagnostic odds ratio) of each Child SCAT5 score were also calculated. Results Concussed children endorsed more symptoms (p < 0.001, $r$ r =0.45), higher symptom severity (p < 0.001, $r$ r =0.44), and had higher double leg (p = 0.046, $r$ r =0.23), single leg (p = 0.035, $r$ r =0.24), and total scores (p = 0.022, $r$ r =0.26) for the mBESS than the non-concussed children. No significant differences were observed for the SAC-C scores (p’s ≥ 0.542). The quantity and severity of endorsed symptoms had the best diagnostic accuracy (AUC = 0.76–0.77), negative predictive values (NPV = 0.84–0.88), and negative likelihood ratios (-LR = 0.22–0.31) of the Child SCAT5 scores. Conclusions Clinicians should prioritize interpretation of the symptom evaluation form of the Child SCAT5 as it was the most effective component for differentiating between concussed and non-concussed middle school children on the same day as a suspected concussion.