The Sun Belt is a region of the United States generally considered stretching across the Southeast and Southwest. Another rough definition of the region is the area south of the Parallel 36°30′ north. Several climates can be found in the region—desert/semi-desert (Eastern California, Nevada, Arizona, New Mexico, Utah, and West Texas), Mediterranean (California), humid subtropical (Alabama, Mississippi, Louisiana, Florida, Georgia, South Carolina, North Carolina, Arkansas,Tennessee and Texas), and tropical (South Florida).

The Sun Belt has seen substantial population growth post-World War II from an influx of people seeking a warm and sunny climate, a surge in retiring baby boomers, and growing economic opportunities. The advent of air conditioning created more comfortable summer conditions and allowed more manufacturing and industry to locate in the Sun Belt. Since much of the construction in the Sun Belt is new or recent, housing styles and design are often modern and open. Recreational opportunities in the Sun Belt are often not tied strictly to one season, and many tourist and resort cities in the region support a tourist industry all year.

Of the 15 fastest-growing cities in the U.S., 12 are located in the Sun Belt as of 2023. Additionally, 86 percent of the top 50 zip codes that saw the largest increases in new residents since the start of the COVID-19 pandemic were in Texas, Florida, and Arizona. The traditional explanations for the growth are increasing productivity in the South and West and increasing demand for Sun Belt amenities, especially its pleasant weather. Job decline in the Rust Belt, along with rising cost of living in several Northeastern states, is another major reason for migration.

The Sun Belt comprises the southern tier of the U.S., including the states of Alabama, Arizona, Florida, Georgia, Louisiana, Mississippi, New Mexico, South Carolina, Texas, roughly two-thirds of California (up to Greater Sacramento), and the southern parts of Arkansas, North Carolina, Nevada, Oklahoma, Tennessee, and Utah. Five of the states—Arizona, California, Florida, Nevada, and Texas—are sometimes collectively called the Sand States because of their abundance of beaches and/or deserts. Other definitions may also include parts of Colorado, Kansas, Missouri, and Virginia and most or all of Arkansas, North Carolina, Oklahoma and Tennessee. For example, the Kinder Institute for Urban Research defines the Sun Belt as being south of 36°30′N latitude, which includes all of Arkansas, most of Oklahoma and virtually all of Tennessee (small parts of East and Middle Tennessee extend north of 36°30′ due to surveying errors) but leaves out most of Nevada and California, with only Southern California and parts of Nye County and the Las Vegas Valley being included. This definition also includes most of the Missouri Bootheel.

First employed by political analyst Kevin Phillips in his 1969 book The Emerging Republican Majority, the term "Sun Belt" became synonymous with the southern third of the nation in the early 1970s. In this period, economic and political prominence shifted from the Midwest and Northeast to the South and West. Factors such as the warmer climate, the migration of workers from Mexico, and a boom in the agriculture industry allowed the southern third of the United States to grow economically. The climate spurred not only agricultural growth, but also the migration of many retirees to retirement communities in the region, especially in Florida and Arizona.

Industries such as aerospace, defense, and oil boomed in the Sun Belt as companies took advantage of the low involvement of labor unions in the region (due to more recent industrialization, 1930s–1950s) and the proximity of military installations that were major consumers of their products. The oil industry helped propel states such as Texas and Louisiana forward, and tourism grew in Florida, and Southern California. More recently, high tech and new economy industries have been major drivers of growth in California, Florida, Texas, and other parts of the Sun Belt. Texas and California rank among the top five states in the nation with the most Fortune 500 companies.

In 2005, the U.S. Census Bureau projected that approximately 88% of the nation's population growth between 2000 and 2030 would occur in the Sun Belt. California, Texas, and Florida were each expected to add more than 12 million people during that time, which would make them by far the most populous states in America. Nevada, North Carolina, Arizona, Colorado, Georgia, Florida, South Carolina, and Texas were expected to be the fastest-growing states.

Events leading up to and including the 2008–2009 recession led some to question whether growth projections for the Sun Belt had been overstated. The economic bubble that led to the recession appeared, to some observers, to have been more acute in the Sun Belt than other parts of the country. Additionally, the traditional lure of cheaper labor markets in the region compared with America's older industrial centers has been eroded by overseas outsourcing trends.