Aim: The NULL-PLEASE score (Nonshockable rhythm, Unwitnessed arrest, Long no-flow or Long low-flow period, blood pH < 7.2, Lactate > 7.0 mmol/L, End-stage renal disease on dialysis, Age ≥85 years, Still resuscitation, and Extracardiac cause) may identify patients with out-of-hospital cardiac arrest (OHCA) unlikely to survive. We aimed to validate the NULL-PLEASE score in a nationwide setting. Methods: We used Danish nationwide registry data from 2001 to 2019 and identified OHCA survivors with return of spontaneous circulation (ROSC) or ongoing cardiopulmonary resuscitation at hospital arrival. The primary outcome was 1-day mortality. Secondary outcomes were 30-day mortality and the combined outcome of 1-year mortality or anoxic brain damage. The risks of outcomes were estimated using logistic regression with a NULL-PLEASE score of 0 as reference (range 0–14). The predictive ability of the score was examined using the area under the receiver operating characteristics (AUC_ROC) curve. Results: A total of 3,881 patients were included in the analyses. One-day mortality was 35%, 30-day mortality was 61%, and 68% experienced the combined outcome. For a NULL-PLEASE score ≥9 (n = 244) the absolute risks were: 1-day mortality: 80.7% (95% confidence interval [CI]: 75.8–85.7%); 30-day mortality: 98.0% (95% CI: 96.2–99.7%); and the combined outcome: 98.4% (95% CI: 96.8–100.0%). Corresponding AUC_ROC values were 0.800 (95% CI: 0.786–0.814) for 1-day mortality, 0.827 (95% CI: 0.814–0.840) for 30-day mortality, and 0.828 (95% CI: 0.815–0.841) for the combined outcome. Conclusions: In a nationwide OHCA-cohort, AUC_ROC values for the predictive ability of NULL-PLEASE were high for all outcomes. However, some survived even with high NULL-PLEASE scores.