We describe an incident we investigated in which a photographer on the Shiretoko Peninsula, Hokkaido, Japan observed a pair of adult brown bears (Ursus arctos yesoensis) mating on 11 October 2009. After an interview and examination of photographs, we conclude that the mating did occur on the reported date. Copulation evidently lasted about 30 minutes without overt struggle either before or after the event. Several days prior to the date, a female, which we presume to be the one involved, was seen with swollen and colored genitalia, suggesting she was in estrus. Only a single mating was witnessed and neither of the bears was observed after this date. Throughout its range, mating of brown bear is rarely observed in autumn, and this is probably the first record in Hokkaido.