Remsen is a city in Plymouth County, Iowa, United States. The population was 1,678 at the time of the 2020 census. Remsen was named for Dr…Remsen is a city in Plymouth County, Iowa, United States. The population was 1,678 at the time of the 2020 census. Remsen was named for Dr. William Remsen Smith. Smith had a good friend who suggested the name Smithville, but Smith thought that there were too many Smiths in the world, so she chose to give the town his middle name.