Red Oak is a city in, and the county seat of, Montgomery County, Iowa, United States, located along the East Nishnabotna River. The population was 5,362 in the 2020 census, a decline from the 6,197 population in 2000.

Montgomery County is a county located in the southwestern area of the U.S. state of Iowa. As of the 2020 census, the population was 10,330. Its population has declined since a peak in 1900, since urbanization and decline of family farms. The county seat is Red Oak. The county was founded by European-American migrants from eastern areas in 1851. It was named in honor of Richard Montgomery, an American Revolutionary War general killed in 1775 while trying to capture Quebec City, Canada.