county, northwestern New York state, U.S. It consists of a lowland region bounded to the north by Lake Ontario (Ontario, Lake), to the west by Ontario, Can. (the Niagara River constituting the border), and to the south by Tonawanda Creek, which is incorporated into the Erie Canal (itself part of the New York State Canal System). Among the other waterways are Power Reservoir and Eighteenmile and Johnson creeks. Niagara Falls, a cataract on the Niagara River between New York and Ontario, is one of North America's best-known natural wonders and an important source of hydroelectric power for both the United States and Canada. The falls are surrounded by parklands—including Niagara Falls State Park, New York's oldest state park (established 1885). Among the other state parks are Whirlpool, Devil's Hole, Wilson-Tuscarora, and Golden Hill. Oak trees predominate in wooded areas.

The county was established in 1808, its name derived from an Iroquoian word meaning “bottomlands.” The principal communities are Niagara Falls, North Tonawanda (Tonawanda–North Tonawanda), Lewiston, and Lockport, which is the county seat. The county contains several military reservations. The economy relies on tourism and heavy industry. Area 523 square miles (1,354 square km). Pop. (1990) 220,756; (2000) 219,846; (2006 est.) 216,130.