Manhattan is a city in and the county seat of Riley County, Kansas, U.S., although the city extends into Pottawatomie County. It is located in n…Manhattan is a city in and the county seat of Riley County, Kansas, U.S., although the city extends into Pottawatomie County. It is located in northeastern Kansas at the junction of the Kansas River and Big Blue River. As of the 2020 Census, the population of the city was 54,100.