Florida, the most populous state in the southeastern US, is also one of the fastest growing of the 50 states. In 1960, it was the 10th most populous state; by 1980, it ranked 7th with a population of 9,746,324; and by 1990, it ranked 4th, with a population of 12,937,926. Between 1990 and 2000, Florida had the 3rd-largest population gain among the states, surpassed only by California and Texas. In that decade, Florida's population grew from 12,937,926 to 15,982,378, an increase of 23.5% (also one of the largest percentage gains in the country). In 2002, Florida had the 4th-largest population of all 50 states, with an estimated total of 16,713,149, a 4.6% increase since 2000. By 2025 Florida is expected to be the 3rd most populous state, with a population of 20.7 million.

The first US census to include Florida, in 1830, recorded a total population of only 34,730. By 1860, on the eve of the Civil War, the population had more than quadrupled, to 140,424 people; about 80% of them lived in the state's northern rim, where cotton and sugarcane plantations flourished. Newcomers migrating southward in the late 19th century through the early 1920s sharply increased the state's population; the 1930 census was the first in which the state passed the million mark. Migration from other states, especially of retirees, caused a population explosion in the post-World War II period, with much of the increase occurring along the south Atlantic coast. From 1950 to 1960, Florida's population increased 79%—the fastest rate of all the states. From 1960 to 1970, the growth rate was 37%; from 1970 to 1980, 44%; from 1980 to 1990, 33%; and from 1990 to 1998, 15.3%.

In 2000, the average population density was 296.4 per sq mi, the eighth highest in the nation. The median age of the population was 38.7. Nearly 22.8% of the population was under age 18 while over 17.6% of was 65 years of age or older, the highest such percentage of all the states and nearly 50% above the US average.

The most populous city in Florida is Jacksonville, the 14th-largest city in the US in 2002. Its population in that year was estimated at 762,461. Miami is Florida's 2nd-largest city, with an estimated 2002 population of 374,791. The Miami-Ft. Lauderdale metropolitan area, the state's largest metropolitan region, had an estimated 3,711,102 residents in 1999; the Jacksonville metropolitan area's population was 1,056,332. Florida's 2nd-largest metropolitan area was Tampa-St. Petersburg-Clearwater, with an estimated 2,278,169 residents; the city of Tampa had an estimated 315,140 people in 2002, and St. Petersburg had 248,546. Ft. Lauderdale had an estimated population of 158,194 in 2002. Tallahassee, the state capital, had a population of 155,171.