This study investigated the potential of the parasitic tachinid fly, Pentatomophaga latifascia (Diptera: Tachinidae), as a biological control agent against adult brown marmorated stink bug, Halyomorpha halys (Hemiptera: Pentatomidae), in Akita Prefecture, northern Japan. Overwintering adults of H. halys were collected from a hibernation site from 2017 to 2019, and the adults were reared at 20 °C in the following spring of each year. Almost all female and male adults parasitised by P. latifascia survived only a few days after the larvae of P. latifascia emerged, and none of the females laid eggs. Emergence of larval P. latifascia from the adults was observed from 18 days after the start of rearing, and all emergence was completed within a 20-day period. The parasitism rate of the field population of overwintered H. halys adults was about 10% each year, and the survival rates of adults of both sexes decreased by about 10%, in synchrony with the time of emergence of larval P. latifascia. These observations indicated that P. latifascia contributed to a decrease in the population density of post-overwintering adult H. halys, at least in early spring.