Ki-67 has been reported as a prognostic marker in human cancers treated using RT. The current study investigated the prognostic significance of Ki-67 expression and its association with clinicopathological characteristics in 19 cats diagnosed with nasal adenocarcinoma and treated using hypofractionated RT. Data collected encompassed signalment, clinical signs, clinicopathological variables, treatment outcomes, and survival times. Median survival times (MST) were estimated using Kaplan–Meier curves and analyzed based on the log-rank test. Based on the results, white blood cell counts were significantly (p < 0.001) higher before treatment than after, although this was not associated with survival time. The overall MST was 550 days (range: 56–1118 days). Cats achieving a favorable response (complete or partial response) had significantly (p = 0.006) longer survival times (1055 days) than those with a poor response (stable or progressive disease; 369 days). Cats with high Ki-67 expression had significantly (p = 0.028) longer survival times (1055 days) than those with low Ki-67 expression (256 days). These results suggested that Ki-67 may be a potential prognostic factor for feline nasal adenocarcinoma.