Niles is a city in Trumbull County, Ohio, United States. The city's population was 20,932 at the 2000 census. It is part of the Youngstown-Warren-Boardman, OH-PA Metropolitan Statistical Area. Located in the nation's former industrial belt, the city's economy focused initially on iron manufacturing but later diversified to include steel and glass production. Niles was adversely affected by the decline of the manufacturing sector throughout the northern United States in the 1970s.