Welcome to the City of New Plymouth, Idaho. New Plymouth is a wonderful community that welcomes everyone who comes to visit. The country is lovely, the people are warm, and life here moves at just the right pace. The city streets create a horseshoe shape, giving the small town the title of the "World's Largest Horseshoe". New Plymouth's population is about 1,535. The Payette County Fairgrounds are located in New Plymouth. New Plymouth is located between The Payette river to the East and the Snake River (Oregon/Idaho border) to the West.