Here, we document the first record for Montana of Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), a major agricultural and nuisance pest in many parts of the world. The insect was found overwintering in a private residence in January 2021 in Billings (Yellowstone Co.). A preliminary survey of sites in Billings from May to October 2021 confirmed the presence of H. halys within a radius of at least 11 km from the January detection. Putatively overwintering H. halys and adult bugs found across the city of Billings in the summer suggest the insect may have been present in the area prior to 2021. Surveys conducted in various regions of the state between 2013 and 2020 reported no detections of H. halys. Yellowstone Co. was previously surveyed in 2017, indicating the species likely arrived between 2018 and 2020 or was present before 2018 at densities too low to be detected. The summary of presence and absence records we provide herein is valuable for future monitoring and management efforts of this economically important species.