Computed tomography (CT) is thought to play a key role in COVID-19 diagnostic work-up. The possibility to compare data across different settings depends on the systematic and reproducible manner the scans are analyzed and reported. The COVID-19 Reporting and Data System (CO-RADS) and the corresponding CT severity score (CTSS) introduced by the Radiological Society of the Netherlands (NVvR) attempt to do so. However, this system has not been externally validated.We aimed to prospectively validate the CO-RADS as a COVID-19 diagnostic tool at the emergency department (ED), and evaluate if the CTSS is associated with prognosis.We conducted a prospective, observational study in two tertiary centers in The Netherlands, between March 19 and May 28, 2020. We consecutively included 741 adult patients at the ED with suspected COVID-19, who received a chest CT and SARS-CoV-2 PCR (PCR). Diagnostic accuracy measures were calculated for CO-RADS using PCR as reference. Logistic regression was performed for CTSS in relation to hospital admission, ICU admission and 30-day mortality.741 patients were included. We found an AUC of 0.91 (CI 0.89-0.94) for CO-RADS using PCR as reference. The optimal CO-RADS cut-off was 4, with a sensitivity of 89.4% (CI 84.7-93.0) and specificity of 87.2% (CI 83.9-89.9). We found a significant association between CTSS and hospital admission, ICU admission, and 30-day mortality; adjusted odds ratios per point increase in CTSS were 1.19 (CI 1.09-1.28), 1.23 (1.15-1.32), 1.14 (1.07-1.22), respectively. Intra-class correlation coefficients for CO-RADS and CTSS were 0.94 (0.91-0.96) and 0.82 (CI 0.70-0.90).Our findings support the use of CO-RADS and CTSS in triage, diagnosis and management decisions for patients presenting with possible COVID-19 at the ED.