Recent studies have indicated that T helper 17 (Th17) cells are involved in the pathogenesis of various inflammatory diseases in dogs. However, age-related changes in canine Th17 cells have not yet been investigated. In the present study, the proportion of Th17 cells was examined in the peripheral blood mononuclear cells (PBMCs) of healthy dogs at various ages: Group 1 (n = 16; less than 1 year of age), Group 2 (n = 25; 1-5 years), and Group 3 (n = 19; 6-9 years), using flow cytometry and an anti-human interleukin (IL)-17A monoclonal antibody that reacts with canine IL-17A. The proportion of circulating Th17 cells positively correlated with age. The age-related differences were observed in the proportion of Th17 cells among Group 1 (mean ± SD: 1.52 ± 1.18%), Group 2 (mean ± SD: 3.81 ± 1.94%) and Group 3 (mean ± SD: 7.49 ± 2.54%). Our results suggest that age-related changes in Th17 cells need to be considered in future research on Th17-related diseases in dogs.