We conducted a serological survey to detect antibodies against influenza A virus (IAV) in Japanese wild boars in Kagoshima prefecture, Japan, between 2014 and 2017. Sero-prevalence against a pandemic-like swine H1N1 (H1N1pdm) virus was identified in 27.1% of specimens, and 1.7% were positive for both swine H1N2 and H3N2 viruses, indicating that wild boars could play an important role in the dynamics of H1N1pdm viral dispersion in the wild. The high frequency of positive results for sera against the H1N1pdm virus suggests that cross-species IAV transmission between wild boars, livestock and humans is a threat to veterinary and public health.