Northumberland County is situated on the north shore of Lake Ontario, in eastern Ontario, Canada. Located east of Durham County (now the Regional Municipality of Durham), west of Hastings, southwest of Kawartha Lakes and south of Peterborough County. The county seat is Cobourg. Together with Durham County, it formed the United Counties of Northumberland and Durham from 1800 to 1973.

It was first aggressively settled by United Empire Loyalists fleeing the former 13 British American Colonies in the late 18th century. The Crown provided plots to the settlers for nominal sums (with the stipulation of making the land productive in a set number of years) or gratis to those who had served against the American Colonial Army.

It consists of:

• Municipality (town) of Brighton
• Town of Cobourg
• Municipality of Port Hope
• Municipality (town) of Trent Hills
• Township of Alnwick/Haldimand
• Township of Cramahe
• Township of Hamilton