Background The Nursing Activities Score (NAS) is widely used for measuring the workload of intensive care unit (ICU) nurses. However, the performance of the NAS to measure actual nursing time has not been comprehensively and externally validated. The aim of this study is to validate the NAS using time-and-motion measurements in Dutch ICUs. Methods We measured nursing time for patients admitted to seven Dutch ICUs, between November 2016 and October 2017. The patient(s) that were under the care of a chosen nurse were followed by the observers during the entire shift and measurements were performed using an in-house developed web application. To validate the reliability of the NAS, we first converted NAS points per activity into minutes. Next, we compared the converted time per NAS item and the converted total nursing time per patient with the actual observed time. We used Wilcoxon signed-rank tests at nursing activity level and Pearson’s R and R2 at patient level for these comparisons. Results A Pearson’s correlation of R=0.59 (R2=0.35) was found between the total converted NAS time and the total observed time per patient. The median converted NAS time per patient (202.6 minutes) was higher compared with the observed time per patient (114.3 minutes). At NAS item level, we found significant differences between the converted NAS time and the observed time for all separate NAS items. Conclusions The NAS overestimates the nursing time needed for patients in Dutch ICUs. Therefore, we advise revisions of the time weighting assigned to each NAS item to obtain better insight into the true nursing workload so that this information can be used for more effective nursing capacity planning. © 2021, Netherlands Society of Intensive Care. All rights reserved.