Allen County is a county in the U.S. state of Indiana. As of the 2020 Census, the population was 385,410, making it the third-most populous county in Indiana. The county seat and largest city is Fort Wayne, the second largest city in Indiana.

Allen County is included in the Fort Wayne Metropolitan Statistical Area and the Fort Wayne–Huntington–Auburn Combined Statistical Area. Allen County is the cultural and economic center of northeastern Indiana. The county is within a 200-mile (320 km) radius of major population centers, including Chicago, Cincinnati, Cleveland, Columbus, Detroit, Indianapolis, Louisville, Milwaukee, and within a one-day drive of one-third of the U.S. population and one-fifth of Canadians.

Occupied for thousands of years by cultures of indigenous peoples, Allen County was organized by European Americans on December 17, 1823, from Delaware and Randolph counties, and formed on April 1, 1824, at the Ewing Tavern. The county is named for Colonel John Allen, an attorney and Kentucky state senator who was killed in the War of 1812. Fort Wayne, founded at the confluences of the Maumee, St. Joseph, and St. Marys rivers was chosen as the county seat in May 1824.

According to the 2010 census, the county has a total area of 660.02 square miles (1,709.4 km²), the largest county in Indiana, of which 657.31 square miles (1,702.4 km²) (or 99.59%) is land and 2.71 square miles (7.0 km²) (or 0.41%) is water.

As of the 2010 United States census, there were 355,329 people, 137,851 households, and 90,892 families residing in the county. The population density was 540.6 inhabitants per square mile (208.7/km²). There were 152,184 housing units at an average density of 231.5 per square mile (89.4/km²). The racial makeup of the county was 79.3% white, 11.7% black or African American, 2.7% Asian, 0.4% American Indian, 0.1% Pacific islander, 2.9% from other races, and 2.9% from two or more races. Those of Hispanic or Latino origin made up 6.5% of the population. In terms of ancestry, 33.4% were German, 11.4% were Irish, 10.7% were American, and 8.1% were English.

Of the 137,851 households, 34.6% had children under the age of 18 living with them, 48.2% were married couples living together, 13.1% had a female householder with no husband present, 34.1% were non-families, and 28.1% of all households were made up of individuals. The average household size was 2.53 and the average family size was 3.12. The median age was 35.3 years.

The median income for a household in the county was $47,697 and the median income for a family was $60,184. Males had a median income of $45,294 versus $33,685 for females. The per capita income for the county was $24,532. About 9.1% of families and 12.3% of the population were below the poverty line, including 17.4% of those under age 18 and 5.8% of those age 65 or over.

Fort Wayne is cited as having the highest Burmese refugee population in the United States, with between 3,000 and 3,500.