Map - Onondaga County, New York (Onondaga County)

Onondaga County is a county in the U.S. state of New York. As of the 2020 census, the population was 476,516. The county seat is Syracuse.

Onondaga County is the core of the Syracuse, NY MSA.

The name Onondaga derives from the name of the Native American tribe who lived in this area at the time of European contact, one of the original Five Nations of the Haudenosaunee. They called themselves (autonym) Onoda'gega, sometimes spelled Onontakeka. The word means "People of the Hills." Sometimes the term was Onondagaono ("The People of the Hills"). The federally recognized Onondaga Nation has a 9.3 sqmi reservation within the county, on which they have self-government.

When counties were established in New York in 1683, the present Onondaga County was part of Albany County. This enormous county included the northern part of New York State as well as all of the present State of Vermont and, in theory, extended westward to the Pacific Ocean. It 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 5 mi 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. The Onondaga were among four Iroquois tribes that allied with the British against the American colonists, as they hoped to end their encroachment. Instead, they were forced to cede most of their land in New York to the United States after the war. Many Onondaga went with Joseph Brant and other nations to Canada, where they received land grants in compensation and formed the Six Nations of the Grand River First Nation.

In 1784, after a peace treaty ended the American Revolutionary War, the name of Tryon County was changed to Montgomery County. It honored General Richard Montgomery, who had captured several places in Canada and died attempting to capture the city of Quebec, and replaced the name of the hated British governor.

In 1789, Montgomery County was reduced by the splitting off of Ontario County from Montgomery. The actual area split off from Montgomery County was much larger than the present county, also including the present Allegany, Cattaraugus, Chautauqua, Erie, Genesee, Livingston, Monroe, Niagara, Orleans, Steuben, Wyoming, Yates, and part of Schuyler and Wayne Counties.

In 1791, Herkimer County was one of three counties split off from Montgomery (the other two being Otsego, and Tioga County). This was much larger than the present county, however, and was reduced by a number of subsequent splits.

In 1794, Onondaga County was split off from Herkimer County. This county was larger than the current Onondaga County, including the present Cayuga, Cortland, and part of Oswego Counties.