AbstractCanine malignant melanoma is a common cancer with a high mortality rate and is a clinically important disease. DNA methylation has been considered to be a potential tumorigenic mechanism through aberrant DNA methylation at promoter region which represses gene transcription. Global hypomethylation could also facilitate chromosome instability. There are few reports regarding DNA methylation in canine malignant melanoma; therefore, the purpose of this study was to examine DNA methylation status of long interspersed nucleotide element‐1 (LINE‐1) to be a surrogate marker of genome‐wide methylation changes in this disease. We measured levels of DNA methylation of two adjacent cytosine‐guanine sites on CpG island (CGI) at the putative promoter of canine LINE‐1 sequence by bisulphite‐pyrosequencing in 41 canine melanoma patient samples as well as six cell lines compared with normal mucosae. The survival rates were obtained from owners or medical records. We found DNA methylation levels of LINE‐1 in normal mucosae were methylated. Interestingly, both melanoma cell lines and clinical melanoma samples showed remarkable hypomethylation. In addition, patients with lower LINE‐1 methylation showed worse prognosis than those with higher LINE‐1 methylation, though the difference did not reach statistical significance (P = .09). Here, we demonstrate that hypomethylation of LINE‐1 is an epigenetically aberrant feature in canine melanoma with possible prognostic value.