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 5,742 in the 2010 census, a decline from the 6,197 population in 2000.
Red Oak derives its name from Red Oak Creek which flows through the community and was noted for the red oaks on its banks. The first settlers arrived there in the 1850s. In 1865 it became the official county seat of Montgomery County and the courthouse which had been in the middle of the county seven miles east in Frankfort, Iowa was towed to the community during a snow storm. That courthouse remained in place until the current Montgomery County Courthouse was built in 1891. It is on the National Register of Historic Places.
In 1869 the community was officially founded when the Chicago, Burlington and Quincy Railroad (called the Burlington and Missouri River Railroad at the time) arrived. The community was named Red Oak Junction by the railroad on March 20, 1876. The junction was a line off the main Chicago to Council Bluffs, Iowa line that went to Nebraska City, Nebraska and on to Lincoln, Nebraska. The Junction name was dropped in 1901 although the community continues to celebrate its history each summer in Junction Days. In 1903 a new depot was opened by the railroad.