Carroll County is a county located in the U.S. state of Indiana. As of the 2020 United States census, the population was 20,306. The county seat is Delphi. Other towns within the county include Flora, Burlington, Camden, Cutler, Bringhurst and Deer Creek. Carroll County is part of the Lafayette, Indiana, Metropolitan Statistical Area.

In 1787, the fledgling United States defined the Northwest Territory, 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 territory's first governor, and Vincennes was established as the territorial 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.

Starting in 1794, Native American titles to Indiana lands were extinguished by usurpation, purchase, or war and treaty. The United States acquired land from the Native Americans in the treaty of St. Mary's in 1818, which included most of the future county, and in 1826 the Treaty of Mississinewas made more area available for settlement. These two treaties resolved the occupation issue for the future Carroll County.

White people had been living in the future Carroll County area since 1824 and their numbers increased rapidly in the next few years. Accordingly, Carroll County was authorized by the state legislature on January 17, 1828; it was named for Charles Carroll, the last surviving signer of the Declaration of Independence, who died in 1832. The first commissioners began organizing the county government on April 28 of that year and determined to site the county seat at land donated by William Wilson. The name 'Carrollton' was first suggested for the new community, but at the suggestion of General Samuel Milroy, 'Delphi' was selected instead.

The Wabash and Erie Canal, built through the county in 1840 and operating until the early 1870s, is among the county's most significant historical legacies.

The first county courthouse was built in 1831. It was replaced by a brick structure in 1856. The 730-pound bell from the first courthouse was made in Cincinnati in 1836 and given by Sheriff Samuel Davis Gresham and was used in the second courthouse until 1916. While the bell was in transit to Lafayette by boat, the boat sank. The bell was recovered from the Ohio River, and was sold, but was returned to Carroll County in 1967.

The current Carroll County courthouse was designed by Jaxson Cowell of Indianapolis, who also designed the Spencer County courthouse. It was constructed by A. E. Kemmer at a cost of about $250,000 from 1916 to 1917. The exterior is understated, but the interior is surprisingly elaborate, including a stained glass dome over a mosaic tile floor.

The Wabash River flows southwestward from Cass County through the upper part of Carroll County, exiting into Tippecanoe County. The Tippecanoe River flows southward through the county's upper west edge, also exiting into Tippecanoe County, where it merges with the Wabash shortly after leaving Carroll County's border. Wildcat Creek drains the south part of Carroll County, flowing westward into Tippecanoe to merge with the Wabash there.