Hendricks County is a county in the U.S. state of Indiana. As of the 2020 United States census, the population was 174,788. The county seat is the town of Danville. Hendricks County is the third most populous county in the Indianapolis-Carmel-Anderson, IN Metropolitan Statistical Area. Hendricks County is currently the second fastest-growing county in Indiana and 85th in the nation.

After the American Revolutionary War established US sovereignty over the territory of the upper midwest, the new federal government defined the Northwest Territory in 1787 which included the area of present-day Indiana. In 1800, Congress separated Ohio from the Northwest Territory, designating the rest of the land as the Indiana Territory. President Thomas Jefferson chose William Henry Harrison as the governor of the territory, and Vincennes was established as the future capital. After the Michigan Territory was separated and the Illinois Territory was formed, Indiana was reduced to its current size and geography. By December 1816 the Indiana Territory was admitted to the Union as a state, although much of its territory was still in dispute as to possession by Native Americans. The land containing Hendricks County was brought into the possession of the United States by the Treaty of St. Mary's in 1818 (known as New Purchase).

The eighth General Assembly of the new state of Indiana met at its then-capital town, Corydon, in December 1823, and created three counties before the close of the session, among them being Hendricks, the state's fifty-first county. The bill creating Hendricks County was introduced in the Senate on December 9 and was signed on December 20 by Indiana Governor William Hendricks, in whose honor the new county was named. Until its governing structure was organized, the territory was assigned to neighboring counties for legislative and enforcement matters.

Prior to the New Purchase treaty, the territory of Hendricks County had been occupied by the Dakota tribe, although there were no significant native villages within its future borders. The treaty authorized the territory to be surveyed by the federal government, with land being available for settlement thereafter. This survey work began in the Hendricks County area in 1819, and settlers began moving in as soon thereafter (the first in future Hendricks County in Spring 1820) as land filing was authorized, such that by 1823 there were sufficient inhabitants to justify forming a county. Designated commissioners met in July 1824, and on July 12 selected Danville as its county seat. The town plat was filed for record in October 1824; a court house was erected and in operation there by 1826.

The county's low rolling hills are entirely devoted to agriculture or urban development, except for wooded drainages. The West Fork of White Lick Creek flows southeastward through the western part of the county and the East Fork flows southward through the east part of the county; the two flows converge at the south county line. The highest point on the terrain (1,030 feet/310 meters ASL) is a small rise 1.5 miles (2.4 km) WNW from Danville.

According to the 2010 United States census, the county has a total area of 408.78 square miles (1,058.7 km²), of which 406.91 square miles (1,053.9 km²) (or 99.54%) is land and 1.87 square miles (4.8 km²) (or 0.46%) is water.

In recent years, average temperatures in Danville have ranged from a low of 18 °F (−8 °C) in January to a high of 86 °F (30 °C) in July, although a record low of −27 °F (−33 °C) was recorded in January 1994 and a record high of 106 °F (41 °C) was recorded in July 1936. Average monthly precipitation ranged from 2.41 inches (61 mm) in February to 4.42 inches (112 mm) in July.

As of the 2020 census, the county had a population of 174,788. The median age was 38.4 years. 25.4% of residents were under the age of 18 and 15.0% of residents were 65 years of age or older. For every 100 females there were 98.5 males, and for every 100 females age 18 and over there were 96.1 males age 18 and over.