Miami-Dade County (/maɪˈæmi ˈdeɪd/), known simply as Dade County prior to 1997, is a county located in the southeastern part of the U.S. state of Florida. The county had a population of 2,701,767 as of the 2020 census, making it the most populous county in Florida and the seventh-most-populous county in the United States. It is Florida's third largest county by land area with 1,946 square miles (5,040 km²). The county seat is Miami, the core of the nation's sixth-largest and world's 65th-largest metropolitan area with a 2020 population of 6.138 million people, exceeding the population of 31 of the nation's 50 states as of 2022.

As of 2022, Miami-Dade County has a gross domestic product of $184.5 billion, making the county's GDP the largest for any county in the State of Florida and the 14th-largest for the nation's 3,033 counties. The county is home to the Port of Miami on Biscayne Bay, the world's largest passenger port with a world record 5.5 million passengers in 2018, and Miami International Airport, the third largest U.S. airport for international passengers and largest U.S. airport for international cargo. The county's land area of nearly 2,000 square miles exceeds that of two U.S. states, Delaware and Rhode Island. The county is home to several universities and colleges, including Florida International University, one of the largest public universities in the country, and the University of Miami in Coral Gables, a private research university that is routinely ranked as one of the nation's top universities and is the county's second-largest employer with nearly 17,000 employees as of 2021.

Miami-Dade County is heavily Hispanic and is the most populous majority-Hispanic county in the nation as of 2020. It is home to 34 incorporated cities and many unincorporated areas. The northern, central and eastern portions of the county are heavily urbanized with many high-rise buildings along the coastline, including Miami's Central Business District in downtown Miami. Southern Miami-Dade County includes the Redland and Homestead areas, which make up the agricultural economy of the county. Agricultural Redland makes up roughly one third of Miami-Dade County's inhabited land area, and is sparsely populated, a stark contrast to the densely populated, urban portions of the county's northern sections.

The county includes portions of two national parks. To the west, the county extends into Everglades National Park and is populated only by a Miccosukee tribal village. Biscayne National Park and the Biscayne Bay Aquatic Preserves are located east of the mainland in Biscayne Bay.

The earliest evidence of Native American settlement in the Miami region is from approximately 12,000 years ago. The first inhabitants settled on the banks of the Miami River, with the main villages on the northern banks.

The inhabitants at the time of first European contact were the Tequesta people, who controlled much of southeastern Florida, including present-day Miami-Dade County, Broward County, and the southern part of Palm Beach County. The Tequesta Indians fished, hunted, and gathered the fruit and roots of plants for food, but did engage in agriculture. They buried the small bones of the deceased with the rest of the body, and put the larger bones in a box for the village people to see. The Tequesta are credited with making the Miami Circle.

Juan Ponce de León was the first European to visit the area in 1513 by sailing into Biscayne Bay. His journal records he reached Chequescha, a variant of Tequesta, which was Miami's first recorded name. It is unknown whether he came ashore or made contact with the natives. Pedro Menéndez de Avilés and his men made the first recorded landing when they visited the Tequesta settlement in 1566 while looking for Avilés' missing son, shipwrecked a year earlier. Spanish soldiers led by Father Francisco Villarreal built a Jesuit mission at the mouth of the Miami River a year later but it was short-lived. After the Spaniards left, the Tequesta Indians were left to fend themselves from European-introduced diseases like smallpox. By 1711, the Tequesta sent a couple of local chiefs to Havana, Cuba, to ask if they could migrate there. The Cubans sent two ships to help them, but Spanish illnesses struck and most of the Tequesta died.

The first permanent European settlers arrived in the early 19th century. People came from the Bahamas to South Florida and the Keys to hunt for treasure from the ships that ran aground on the treacherous Great Florida Reef. Some accepted Spanish land offers along the Miami River. At about the same time, the Seminole Indians arrived, along with a group of runaway slaves. The area was affected by the Second Seminole War, during which Major William S. Harney led several raids against the Indians. Most non-Indian residents were soldiers stationed at Fort Dallas. It was the most devastating Indian war in American history, causing almost a total loss of population in Miami.