|County of Northumberland|
|Coordinates: 44°07′N78°02′W / 44.117°N 78.033°W Coordinates: 44°07′N78°02′W / 44.117°N 78.033°W|
|Provinces and territories of Canada||Ontario|
|• Type||Upper tier municipal government|
|• Total||1,905.15 km2 (735.58 sq mi)|
|• Density||47.78/km2 (123.7/sq mi)|
|Time zone||UTC−05:00 (EST)|
|• Summer (DST)||UTC−05:00 (EDT)|
Northumberland County is an upper-tier level of municipal government situated on the north shore of Lake Ontario, east of Toronto in Central Ontario. The Northumberland County headquarters are located in Cobourg.
Northumberland County consists of seven municipalities (in population order):
The Alderville First Nation is within the Northumberland census division but is independent of county administration.
As a census division in the 2021 Census of Population conducted by Statistics Canada, Northumberland County had a population of 89,365 living in 37,328 of its 40,638 total private dwellings, a change of 4.4% from its 2016 population of 85,598. With a land area of 1,907.4 km2 (736.5 sq mi), it had a population density of 46.9/km2 (121.3/sq mi) in 2021. 
|Population||89,365 (+4.4% from 2016)||85,598 (4.2% from 2011)||82,126 (1.4% from 2006)|
|Land area||1,907.40 km2 (736.45 sq mi)||1,905.15 km2 (735.58 sq mi)||1,905.34 km2 (735.66 sq mi)|
|Population density||46.9/km2 (121/sq mi)||44.9/km2 (116/sq mi)||43.1/km2 (112/sq mi)|
|Median age||52.4 (M: 50.8, F: 54)||51.4 (M: 50.3, F: 52.4)|
|Total private dwellings||37,330||39,032||37,226|
|Median household income||$70,208|
|     |
1996 population adjusted to 2001 boundaries
| Panethnic |
|2021 ||2016 ||2011 ||2006 ||2001 |
|European [lower-alpha 1]||80,305||92.01%||78,215||93.82%||75,835||95.23%||75,710||95.75%||73,210||96.41%|
|East Asian [lower-alpha 2]||810||0.93%||690||0.83%||460||0.58%||495||0.63%||540||0.71%|
|Southeast Asian [lower-alpha 3]||545||0.62%||320||0.38%||250||0.31%||195||0.25%||65||0.09%|
|Middle Eastern [lower-alpha 4]||240||0.27%||80||0.1%||85||0.11%||95||0.12%||65||0.09%|
|Other [lower-alpha 5]||400||0.46%||250||0.3%||210||0.26%||145||0.18%||65||0.09%|
|Note: Totals greater than 100% due to multiple origin responses|
Together with Durham County, it formed the Newcastle District from 1802 to 1849 and the United Counties of Northumberland and Durham from 1850 to 1973. Effective January 1, 1974, part of Durham County was merged with Ontario County to create the Regional Municipality of Durham. At that time, Northumberland reverted to a standalone county.
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 free to those who had served against the American Colonial Army. Following the War of 1812, many port towns, Port Hope and Cobourg in particular, became important centres for commercial activity and a landing point for European immigrants arriving on steamers.
There are two provincial parks in Northumberland County: Presqu'ile Provincial Park in Brighton, and Ferris Provincial Park in Campbellford.  There are also several other protected natural areas and forests, including Ganaraska Forest,  Northumberland County Forest, Goodrich-Loomis Conservation Area, and Peter's Woods. Waterfront campsites and cottages are located along Lake Ontario and Rice Lake.
Northumberland County has various cycling, hiking and other outdoor trails. The Waterfront Trail along Lake Ontario passes through Northumberland County,  as does the Trans-Canada Trail.  The Northumberland portion of the Trans-Canada Trail spans from Hastings to Hoard's Station in Campbellford, following an abandoned rail line. Halfway through Campbellford, the trail joins the 6 km long Rotary Trail situated alongside the Trent River. There are five signed bike routes: Glorious Ganaraska, Rice Lake Ramble, Shelter Valley, Presqu’ile Promise and Trent River Truckin'.  The Northumberland County Forest offers various trails available for hiking, cycling, horseback riding, ATVing, off-road motorcycling, cross-country skiing, snowmobiling, snowshoeing  as well as a 3.2 km, accessible Universal Trail.  The Ganaraska Hiking Trail starts in Port Hope and goes north towards the Bruce Trail. 
There are three theatres in Northumberland County: Westben in Campbellford, the Capitol Theatre in Port Hope, and the Park Theatre & Performing Arts Centre in Cobourg.  Festivals in Northumberland include the Warkworth Maple Syrup and Float Your Fanny Down the Ganny festivals in early spring, the Cobourg Sandcastle Festival and Incredible Edibles Festival in the summer, and the Cultivate Food and Vintage Film Festivals in the fall. 
Whitby is a town in Durham Region. Whitby is located in Southern Ontario east of Ajax and west of Oshawa, on the north shore of Lake Ontario and is home to the headquarters of Durham Region. It had a population of 138,501 at the 2021 census. It is approximately 20 km (12 mi) east of the eastern border of Toronto, and it is known as a commuter suburb in the Greater Toronto Area. While the southern portion of Whitby is predominantly urban and an economic hub, the northern part of the municipality is more rural and includes the communities of Ashburn, Brooklin, Myrtle, Myrtle Station, and Macedonian Village.
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The Township of Alnwick/Haldimand is a township in central Ontario, Canada, in Northumberland County, situated between Lake Ontario and Rice Lake. It was formed in 2001 by the merger of Alnwick Township in the north and Haldimand Township in the south. Alderville First Nation is an autonomously governed First Nation contained within the township boundaries, in two non-contiguous sections along County Roads 45 and 18.
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