The effect of the semi-quantitative expression of CD20 in the prognosis of feline nasal lymphoma has not been described. This study investigated the prognostic significance of CD20 expression, clinicopathological characterization, and treatment outcomes in cats with nasal lymphoma. Clinical data from cats diagnosed with nasal lymphoma were retrospectively collected, including signalment, clinical signs, clinicopathological variables, treatment outcomes, and survival times. Using ImageJ software, CD20 expression was semi-quantitatively measured based on the proportion of CD20-positive areas. Correlations between laboratory findings, immunohistochemical expressions, and survival outcomes were investigated. All cats included in the study exhibited the B-cell immunophenotype. During treatment, a reduction in PCV was noted in the cats at the second and sixth weeks (p = 0.01 and p = 0.01, respectively). The cats with low CD20 expression exhibited a significantly shorter MST (91 days; 95% CI, 41–141) than those with high CD20 expression (MST, 214 days; 95% CI, 76–351) (p = 0.01). Stage T1 cats displayed a higher MST (143 days; 95% CI, 144–172) than those in other stages > T1 (120 days, 95% CI, 71–169 days) (p = 0.04). Anemia, a common adverse effect in feline nasal lymphoma, did not impact MST. T1 clinical staging and high CD20 expression showed a trend for better MST.