Jefferson County is a county in the Commonwealth of Pennsylvania. As of the 2020 census, the population was 44,492. Its county seat is Brookville. The county was established on March 26, 1804, from part of Lycoming County and later organized in 1830. It is named for President Thomas Jefferson. It is home to Punxsutawney Phil, the most famous groundhog that predicts when spring will come every February 2 on Groundhog Day. The county is part of the North Central Pennsylvania region of the state.

According to the U.S. Census Bureau, the county has a total area of 657 square miles (1,700 km), of which 652 square miles (1,690 km) is land and 4.4 square miles (11 km) (0.7%) is water. It has a warm-summer humid continental climate (Dfb) and average monthly temperatures in Brookville range from 24.8 °F in January to 68.8 °F in July, while in Punxsutawney they range from 25.7 °F in January to 69.6 °F in July. Jefferson County is one of the 423 counties served by the Appalachian Regional Commission, and it is identified as part of the "Midlands" by Colin Woodard in his book American Nations: A History of the Eleven Rival Regional Cultures of North America.

• Forest County (northwest)
• Elk County (northeast)
• Clearfield County (east)
• Indiana County (south)
• Armstrong County (southwest)
• Clarion County (west)

There are many named streams flowing through Jefferson County, far too many to list here. An exhaustive list is presented in a separate article. All those streams eventually flow to the Allegheny River except for Bear Run, 1.36 square miles (3.5 km), in the southeast which flows into the Susquehanna River. The principal streams and the area of their watersheds that lie within the county are:

• Clarion River: 130.50 square miles (338.0 km)
  • Little Toby Creek: 34.46 square miles (89.3 km)
  • Clear Creek: 20.12 square miles (52.1 km)
  • Cathers Run: 15.69 square miles (40.6 km)
  • Mill Creek: 30.70 square miles (79.5 km)
• Redbank Creek: 375.52 square miles (972.6 km)
  • North Fork Creek: 91.63 square miles (237.3 km)
    • Clear Run: 9.79 square miles (25.4 km)
    • Pekin Run: 10.12 square miles (26.2 km)
  • Sandy Lick Creek: 143.98 square miles (372.9 km)
    • Wolf Run: 18.19 square miles (47.1 km)
    • Soldier Run: 11.09 square miles (28.7 km)
    • Trout Run: 10.74 square miles (27.8 km)
    • Mill Creek: 53.69 square miles (139.1 km)
    • Fivemile Run: 17.55 square miles (45.5 km)
    • Falls Creek: 11.39 square miles (29.5 km)
  • Little Sandy Creek: 70.65 square miles (183.0 km)
• Mahoning Creek: 149.63 square miles (387.5 km)
  • East Branch Mahoning Creek: 15.27 square miles (39.5 km)
  • Stump Creek: 14.86 square miles (38.5 km)
  • Big Run: 19.41 square miles (50.3 km)
  • Canoe Creek: 11.57 square miles (30.0 km)
  • Elk Run: 13.20 square miles (34.2 km)
  • Pine Run: 18.88 square miles (48.9 km)

Historical population

Census	Pop.	Note	%±
1810	161		—
1820	561		248.4%
1830	2,025		261.0%
1840	7,253		258.2%
1850	13,518		86.4%
1860	18,270		35.2%
1870	21,656		18.5%
1880	27,935		29.0%
1890	44,005		57.5%
1900	59,113		34.3%
1910	63,090		6.7%
1920	62,104		−1.6%
1930	52,114		−16.1%
1940	54,090		3.8%
1950	49,147		−9.1%
1960	46,792		−4.8%
1970	43,695		−6.6%
1980	48,303		10.5%
1990	46,083		−4.6%
2000	45,932		−0.3%
2010	45,200		−1.6%
2020	44,492		−1.6%

As of the census of 2000, there were 45,932 people, 18,375 households, and 12,862 families residing in the county. The population density was 70 people per square mile (27 people/km). There were 22,104 housing units at an average density of 34 units per square mile (13/km). The racial makeup of the county was 98.97% White, 0.13% Black or African American, 0.16% Native American, 0.21% Asian, 0.01% Pacific Islander, 0.07% from other races, and 0.45% from two or more races. 0.41% of the population were Hispanic or Latino of any race. 31.1% were of German, 13.4% Italian, 10.8% American, 9.2% Irish and 7.8% English ancestry.

There were 18,375 households, out of which 30.30% had children under the age of 18 living with them, 56.80% were married couples living together, 9.10% had a female householder with no husband present, and 30.00% were non-families. 26.60% of all households were made up of individuals, and 13.80% had someone living alone who was 65 years of age or older. The average household size was 2.45 and the average family size was 2.96.

In the county, the population was spread out, with 23.60% under the age of 18, 7.70% from 18 to 24, 27.20% from 25 to 44, 23.60% from 45 to 64, and 17.90% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 95.70 males. For every 100 females age 18 and over, there were 92.60 males.

Jefferson County Racial Composition

Race	Num.	Perc.
White (NH)	42,317	95.1%
Black or African American (NH)	125	0.3%
Native American (NH)	45	0.1%
Asian (NH)	100	0.2%
Pacific Islander (NH)	8	0.02%
Other/Mixed (NH)	1,500	3.4%
Hispanic or Latino	397	1%

Jefferson County is a very safe county for the Republican Party, which has won the vote of all but two presidential elections.

United States presidential election results for Jefferson County, Pennsylvania

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2020	17,964	78.54%	4,529	19.80%	379	1.66%
2016	15,192	77.53%	3,650	18.63%	753	3.84%
2012	13,048	71.78%	4,787	26.33%	343	1.89%
2008	12,057	63.75%	6,447	34.09%	408	2.16%
2004	13,371	68.36%	6,073	31.05%	116	0.59%
2000	11,473	65.24%	5,566	31.65%	547	3.11%
1996	8,156	49.56%	5,846	35.52%	2,455	14.92%
1992	7,271	41.05%	5,998	33.87%	4,442	25.08%
1988	9,743	60.48%	6,235	38.71%	131	0.81%
1984	11,334	65.31%	5,950	34.28%	71	0.41%
1980	9,628	57.38%	6,296	37.53%	854	5.09%
1976	9,437	55.09%	7,456	43.53%	237	1.38%
1972	11,631	68.92%	5,024	29.77%	221	1.31%
1968	10,214	55.67%	6,839	37.28%	1,294	7.05%
1964	8,373	43.47%	10,851	56.34%	37	0.19%
1960	13,845	63.82%	7,811	36.01%	38	0.18%
1956	13,051	66.26%	6,627	33.64%	19	0.10%
1952	11,833	64.61%	6,365	34.75%	116	0.63%
1948	9,395	61.43%	5,632	36.82%	268	1.75%
1944	10,970	62.52%	6,425	36.62%	152	0.87%
1940	12,081	58.40%	8,559	41.37%	47	0.23%
1936	11,943	51.24%	11,080	47.54%	283	1.21%
1932	8,246	52.48%	6,570	41.81%	897	5.71%
1928	13,233	74.63%	4,325	24.39%	173	0.98%
1924	10,673	64.88%	2,664	16.19%	3,113	18.92%
1920	7,970	66.69%	3,060	25.61%	920	7.70%
1916	4,332	51.74%	3,253	38.85%	788	9.41%
1912	1,608	18.65%	2,510	29.11%	4,504	52.24%
1908	5,652	60.17%	2,986	31.79%	755	8.04%
1904	5,860	69.09%	2,095	24.70%	527	6.21%
1900	5,950	62.38%	3,063	32.11%	525	5.50%
1896	5,500	57.18%	3,671	38.16%	448	4.66%
1892	4,100	50.41%	3,251	39.97%	782	9.62%
1888	4,090	53.48%	3,257	42.59%	301	3.94%

As of July 22, 2024, there are 27,731 registered voters in Jefferson County.

• Republican: 18,817 (67.86%)
• Democratic: 5,809 (20.94%)
• Independent: 2,096 (7.56%)
• Third Party: 1,009 (3.64%)

• Herbert L. Bullers Jr.(R)
• Scott E. North(R)
• Jeffrey E. Pisarcik(D)

• Cris Dush, District 25
• Joey Pittman, District 41

• Brian Smith, Republican, Pennsylvania's 66th Representative District

• G.T. Thompson, Republican, Pennsylvania's 15th congressional district

• John Fetterman, Democrat
• Bob Casey Jr., Democrat

• Indiana University of Pennsylvania
• Clarion University of Pennsylvania
• Butler County Community College (Brockway)

• Brockway Area School District
• Brookville Area School District
• Clarion-Limestone Area School District
• DuBois Area School District
• Punxsutawney Area School District

• Jefferson County-DuBois AVTS
• Riverview Intermediate Unit #6

• Allens Mills School - Reynoldsville
• Bear Lane School - Punxsutawney
• Blose Hill Amish School - Reynoldsville
• Bucks Run - Reynoldsville
• Canoe Ridge Amish School - Rossiter
• Christ Dominion Academy
• Colonial Drake - Punxsutawney
• Eagles Nest Amish School - Brockway
• Highland Park - Punxsutawney
• Hillside School - Punxsutawney
• Lone Maple School - Punxsutawney
• Maple Grove School - Reynoldsville
• Mountain View School - Punxsutawney
• Munderf Amish School - Brockway
• Oak Grove Parochial School - Smicksburg
• Pine Valley Parochial School - Punxsutawney
• Playhouse Children's Center - Punxsutawney
• Praise Christian Academy - Reynoldsville
• Punxsutawney Christian Sch - Ele Level - Punxsutawney
• Spring Hollow Amish School - Reynoldsville
• Spring Run School - Smickburg
• Sts Cosmas & Damian School - Punxsutawney
• Trout Run School - Punxsutawney
• Valley View School - Punxsutawney
• West Creek Road Amish School - Punxsutawney
• Willow Drive School - Punxsutawney
• Windy Hollow Amish School - Mayport

• Jefferson County Library System - Brockway
• Mengle Memorial Library - Brockway
• Punxsutawney Memorial Library - Punxsutawney
• Rebecca M Arthurs Memorial Library - Brookville
• Reynoldsville Public Library - Reynoldsville
• Summerville Public Library - Summerville
• Sykesville Public Library - Sykesville

• Full Circle Inc Boys Home - Reynolds
• Jefferson County Adult Detention Center
• Western Pennsylvania School of Taxidermy - Oliveburg

Two Pennsylvania state parks are in the county.

• Clear Creek State Park is in Barnett and Heath Townships.
• Cook Forest State Park is in Barnett Township and stretches into neighboring Clarion and Forest Counties

The Jefferson County Fair is held annually in July.

Under Pennsylvania law, there are four types of incorporated municipalities: cities, boroughs, townships, and, in at most two cases, towns. The following boroughs and townships are located in Jefferson County:

• Crenshaw

The population ranking of the following table is based on the 2010 census of Jefferson County.

† county seat

Rank	City/Town/etc.	Municipal type	Population (2020 Census)
1	Punxsutawney	Borough	5,769
2	† Brookville	Borough	3,995
3	Reynoldsville	Borough	2,549
4	Brockway	Borough	2,276
5	Sykesville	Borough	1,115
6	Falls Creek (partially in Clearfield County)	Borough	994
7	Big Run	Borough	647
8	Summerville	Borough	504
9	Crenshaw	CDP	468
10	Corsica	Borough	319
11	Timblin	Borough	147
12	Worthville	Borough	80

• John T. Morrison, sixth Governor of Idaho from 1903 until 1905; born in Jefferson County.
• Sparky Lyle, professional baseball player
• Chuck Daly, American basketball head coach
• George Jenks, politician
• Florence Parry Heide, author
• Wilbur Good, professional baseball player
• Andy Hastings, professional football player
• Mal Eason, professional baseball player
• John Mizerock, professional baseball player
• Devin Mesoraco, professional baseball player
• Jim Pittsley, professional baseball player
• Britt Baker, professional wrestler and dentist
• Laura Temple, missionary teacher and archaeologist in Mexico

• National Register of Historic Places listings in Jefferson County, Pennsylvania
• Oil Creek Library District

41°08′N 79°00′W / 41.13°N 79.00°W / 41.13; -79.00