Although interindividual differences in narcissism are well studied, little is known about assessing narcissism at the within-person level. To fill this research gap, we investigated whether the narcissism construct is represented in the same way at the between- and within-person levels. We analyzed four established narcissism measures across multiple studies. In each of the studies, participants completed narcissism measures in ecological momentary assessment or daily diary studies. Equivalent construct representation across between- and within-person narcissism (i.e., cross-level measurement invariance) was found. State narcissism measures showed convergent validities for the trait narcissism scales. Moreover, we also found that antagonistic narcissism was most strongly related to within-person variability in narcissism. Our investigation sheds new light on the structure and assessment of narcissism on the within-person level by providing a comprehensive examination of its measurement. (PsycInfo Database Record (c) 2024 APA, all rights reserved).