Post Falls is a city in Kootenai County, Idaho, United States. It is the gateway city to Northern Idaho off of Interstate 90, just west of Coeu…Post Falls is a city in Kootenai County, Idaho, United States. It is the gateway city to Northern Idaho off of Interstate 90, just west of Coeur d'Alene, and east of Spokane, Washington. The population was 38,485 at the 2020 census, making it Idaho's ninth-largest city and the second largest city in North Idaho behind Coeur d’Alene.