Background and Aims: The COVID-19 pandemic, caused by the novel coronavirus SARS-CoV-2, has fundamentally reshaped the landscape of global public health, with some people suffering more adverse clinical outcomes than others. The aim of this study is to deepen our understanding of the specific impact of acute kidney injury (AKI) on the in-hospital mortality in octogenarian patients with COVID-19. Methods: This is a prospective observational cohort study, which involved 23 COVID-19 hospital units in the Campania Region, Italy. Exposure variables were collected during hospital admission and at discharge. Only patients aged ≥80 years were deemed eligible for the study. Results: 197 patients were included in the study (median age 83.0 [82.0–87.0] years; 51.5% men), with a median duration of hospitalization of 15.0 [8.0–25.0] days. From the multivariable Cox regression analysis, after the application of Šidák correction, only the respiratory rate (HR 1.09, 95% CI: 1.04 to 1.14; p < 0.001) and AKI development (HR: 3.40, 95% CI: 1.80 to 6.40; p < 0.001) were independently associated with the primary outcome. Moreover, the Kaplan–Meier analysis showed a significantly different risk of in-hospital mortality between patients with and without AKI (log-rank: <0.0001). Conclusions: In our investigation, we identified a significant association between AKI and mortality rates among octogenarian patients admitted for COVID-19. These findings raise notable concerns and emphasize the imperative for vigilant monitoring of this demographic cohort.