Preble County is a county located in the state of Ohio, United States. According to the 2010 census, it has a population of 42,270, which is a decrease of 0.2% from 42,337 in 2000. Its county seat is Eaton. It is named for Edward Preble, a naval officer who fought in the American Revolutionary War and against the Barbary Pirates.

Preble County is part of the Dayton Metropolitan Statistical Area.

According to the 2010 census, the county has a total area of 426.46 square miles (1,104.5 km²), of which 424.12 square miles (1,098.5 km²) (or 99.45%) is land and 2.34 square miles (6.1 km²) (or 0.55%) is water.

Preble County was formed on February 15, 1808 from portions of Butler County and Montgomery County. It named after Edward Preble, a marine officer in the American Revolutionary War.

As of the census of 2000, there were 42,337 people, 16,001 households, and 12,144 families residing in the county. The population density was 100 people per square mile (38/km²). There were 17,186 housing units at an average density of 40 per square mile (16/km²). The racial makeup of the county was 98.47% White, 0.32% Black or African American, 0.21% Native American, 0.26% Asian, 0.02% Pacific Islander, 0.11% from other races, and 0.60% from two or more races. 0.43% of the population were Hispanic or Latino of any race.

Ohio (ⁱ/oʊˈhaɪ.oʊ/) is a state in the Midwestern United States. Ohio is the 34th most extensive, the 7th most populous, and the 10th most densely populated of the 50 United States. The state's capital and largest city is Columbus.

The name "Ohio" originated from Iroquois word ohi-yo’, meaning "great river". The state, originally partitioned from the Northwest Territory, was admitted to the Union as the 17th state (and the first under the Northwest Ordinance) on March 1, 1803. Although there are conflicting narratives regarding the origin of the nickname, Ohio is historically known as the "Buckeye State" (relating to the Ohio buckeye tree) and Ohioans are also known as "Buckeyes".

The government of Ohio is composed of the executive branch, led by the Governor; the legislative branch, which comprises the Ohio General Assembly; and the judicial branch, which is led by the Supreme Court. Currently, Ohio occupies 18 seats in the United States House of Representatives. Ohio is known for its status as both a swing state and a bellwether in national elections.