Miami-Dade County is an American county located in the southeastern part of the U.S. state of Florida. It is the southeastern most county on the U.S. mainland. According to a 2015 census report, the county had a population of 2,693,117 making it the most populous county in Florida and the seventh-most populous county in the United States. It is also Florida's third-largest county in terms of land area, with 1,946 square miles. The county seat is Miami.
Dade County was created on January 18, 1836, under the Territorial Act of the United States. The county was named after Major Francis L. Dade, a soldier killed in 1835 in the Second Seminole War, at what has since been named the Dade Battlefield. At the time of its creation, Dade County included the land that now contains Palm Beach and Broward counties, together with the Florida Keys from Bahia Honda Key north and the land of present-day Miami-Dade County. The county seat was originally at Indian Key in the Florida Keys; then in 1844, the County seat was moved to Miami.
The Florida Keys from Key Largo to Bahia Honda were returned to Monroe County in 1866. In 1888 the county seat was moved to Juno, near present-day Juno Beach, Florida, returning to Miami in 1899. In 1909, Palm Beach County was formed from the northern portion of what was then Dade County, and then in 1915, Palm Beach County and Dade County contributed nearly equal portions of land to create what is now Broward County. There have been no significant boundary changes to the county since 1915.