© 2019 Elsevier B.V. Introduction: There are no established risk classification for post-cardiac arrest syndrome (PCAS) patients at the Emergency Department (ED) undergoing targeted temperature management (TTM). The aim of this study was to externally validate a simplified version of our prognostic score, the “post-Cardiac Arrest Syndrome for Therapeutic hypothermia score” (revised CAST [rCAST]) and estimate the predictive accuracy of the risk classification based on it. Methods: For the external validation, we used data from an out-of-hospital cardiac arrest (OHCA) registry of the Japanese Association for Acute Medicine (JAAM), which is a multicenter, prospective registry of OHCA patients across Japan. Eligible patients were PCAS patients treated with TTM at 33–36 °C between June 2014 and December 2015. We validated the accuracy of rCAST for predicting the neurological outcomes at 30 and 90 days. Results: Among the 12,024 OHCA patients, the data of 460 PCAS patients treated by TTM were eligible for the validation. The areas under the curve of rCAST for predicting the neurological outcomes at 30 and 90 days were 0.892 and 0.895, respectively. The estimated sensitivity and specificity of the risk categories for the outcomes were as follows: 0.95 (95% CI: 0.92–0.98) and 0.47 (0.40–0.55) for the low (rCAST: ≤5.5), 0.62 (0.56–0.68) and 0.48 (0.40–0.55) for the moderate (rCAST: 6.0–14.0), and 0.57 (0.51–0.63) and 0.95 (0.91–0.98) for the high severity category (rCAST: ≥14.5). Conclusions: The rCAST was useful for predicting the neurological outcomes with high accuracy in PCAS patients, and the three grades was developed for a risk classification based on the rCAST.