Niagara County is a county located in the U.S. state of New York. As of the 2000 census, the population was 219,846. The county seat is Lockport. The county name is from the Indian word Ongiara, meaning "thunder of waters," referring to the sound of the famous waterfalls. It is the location of Niagara Falls and Fort Niagara, as has many parks and lake shore recreation communities.

When counties were established in the New York colony in 1683, the present Niagara County was part of Albany County. This was an enormous county, including the northern part of New York State as well as all of the present State of Vermont and, in theory, extending westward to the Pacific Ocean.

This county was reduced in size on July 3, 1766 by the creation of Cumberland County, and further on March 16, 1770 by the creation of Gloucester County, both containing territory now in Vermont. On March 12, 1772, what was left of Albany County was split into three parts, one remaining under the name Albany County. One of the other pieces, Tryon County, contained the western portion (and thus, since no western boundary was specified, theoretically still extended west to the Pacific).

The eastern boundary of Tryon County was approximately five miles west of the present city of Schenectady, and the county included the western part of the Adirondack Mountains and the area west of the West Branch of the Delaware River. The area then designated as Tryon County now includes 37 counties of New York State. The county was named for William Tryon, colonial governor of New York.

In the years prior to 1776, most of the Loyalists in Tryon County fled to Canada. In 1784, following the peace treaty that ended the American Revolutionary War, the name of Tryon County was changed to honor the general, Richard Montgomery, who had captured several places in Canada and died attempting to capture the city of Quebec, replacing the name of the hated British governor.

In 1789, Ontario County was split off from Montgomery. In turn, Genesee County was created from Ontario County in 1802.Niagara County was created from Genesee County in 1808. It was, however, larger than the present Niagara County even though it consisted of only the Town of Cambria.From 1814 to 1817, records of Cattaraugus County were divided between Belmont (the seat of Allegany County) and Buffalo (then in Niagara County). In 1821, Erie County was created from Niagara County.