DNA methylation is an epigenetic mechanism controlling gene expression without affecting DNA sequences, and aberrant DNA methylation patterns are features of a number of diseases. Notably, epigenetic errors in cancer cells have been intensively studied over the last two decades in humans; however, little is known concerning dogs and cats. To analyze DNA methylation and gene expression changes in feline lymphoma cells, we added the DNA methylation inhibitor 5-aza-2′-deoxycytidine (5-aza) to three cell lines (3281 and FT-1 cells derived from T-cell lymphoma and MS4 cells derived from B-cell lymphoma). Adding 5-aza significantly retarded cell growth in a dose-dependent manner in all cell lines, and there were aberrant gene expression patterns. Transcription factor Sox11 expression in 3281 cells was de-repressed by 5-aza treatment, and subsequent promoter DNA demethylation was analyzed by bisulfite sequencing. Cell cycle analysis suggested that inhibition of cell growth was due to DNA replication arrest, and this supported the result of increased expression of p27kip1 gene which disturbed cells of 3281 and FT-1 entering the S phase. In this study, 5-aza suppressed the growth of feline lymphoma cells, but further experiments with normal lymph cells are necessary to confirm specificity of this drug treatment and to expand it for clinical use.