Ismay is a town in Custer County, Montana, United States. The population was 17 at the 2020 census. The town is the least-populous incorpora…Ismay is a town in Custer County, Montana, United States. The population was 17 at the 2020 census. The town is the least-populous incorporated municipality in the state of Montana. The town's name is an amalgamation of Isabella and May, the names of the daughters of Albert J Earling, division superintendent of the Chicago, Milwaukee and St. Paul Railway. Other communities with related names include Earling, Iowa; Marmarth, North Dakota; and Alberton, Montana. As a publicity stunt coordinated by the Kansas City Chiefs in 1993, the town unofficially took the name of Joe, Montana, after the NFL quarterback Joe Montana.