Background: Our understanding of adaptive immune responses in patients with coronavirus disease 2019 (COVID-19) is rapidly evolving, but information on the innate immune responses by natural killer (NK) cells is still insufficient. Objective: We aimed to examine the phenotypic and functional status of NK cells and their changes during the course of mild and severe COVID-19. Methods: We performed RNA sequencing and flow cytometric analysis of NK cells from patients with mild and severe COVID-19 at multiple time points in the course of the disease using cryopreserved PBMCs. Results: In RNA-sequencing analysis, the NK cells exhibited distinctive features compared with healthy donors, with significant enrichment of proinflammatory cytokine-mediated signaling pathways. Intriguingly, we found that the unconventional CD56(dim) CD16(neg) NK-cell population expanded in cryopreserved PBMCs from patients with COVID-19 regardless of disease severity, accompanied by decreased NK cell cytotoxicity. The NK-cell population was rapidly normalized alongside the disappearance of unconventional CD56(dim)CD16(neg) NK cells and the recovery of NK-cell cytotoxicity in patients with mild COVID-19, but this occurred slowly in patients with severe COVID-19. Conclusions: The current longitudinal study provides a deep understanding of the NK-cell biology in COVID-19.