Arizona ( ARR-iz-OH-nə; Navajo: Hoozdo Hahoodzo [hoː˥z̥to˩ ha˩hoː˩tso˩]; O'odham: Alĭ ṣonak [ˈaɭi̥ ˈʂɔnak]) is a state in the Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. Its other neighboring states are Nevada to the northwest and California to the west. It also shares an international border with the Mexican states of Sonora and Baja California to the south and southwest. It is the 6th-largest and the 14th-most-populous of the 50 states. Its capital and largest city is Phoenix, which is the most populous state capital in the United States.

Arizona is the 48th state and last of the contiguous states to be admitted to the Union, achieving statehood on February 14, 1912. Historically part of the territory of Alta California and Nuevo México in New Spain, it became part of independent Mexico in 1821. After being defeated in the Mexican–American War, Mexico ceded much of this territory to the United States in 1848, where the area became part of the territory of New Mexico. The southernmost portion of the state was acquired in 1853 through the Gadsden Purchase.

Southern Arizona is known for its desert climate, with very hot summers and mild winters. Northern Arizona features forests of pine, Douglas fir, and spruce trees; the Colorado Plateau; mountain ranges (such as the San Francisco Mountains); as well as large, deep canyons, with much more moderate summer temperatures and significant winter snowfalls. There are ski resorts in the areas of Flagstaff, Sunrise, and Tucson. In addition to the internationally known Grand Canyon National Park, which is one of the world's seven natural wonders, there are several national forests, national parks, and national monuments.

Arizona is home to a diverse population. About one-quarter of the state is made up of Indian reservations that serve as the home of 27 federally recognized Native American tribes, including the Navajo Nation, the largest in the state and the country, with more than 300,000 citizens. Since the 1980s, the proportion of Hispanics has grown significantly owing to migration from Mexico and Central America. A substantial portion of the population are followers of the Roman Catholic Church and the Church of Jesus Christ of Latter-day Saints. Arizona's population and economy have grown dramatically since the 1950s because of inward migration, and the state is now a major hub of the Sun Belt. Cities such as Phoenix and Tucson have developed large, sprawling suburban areas. Many large companies, such as PetSmart and Circle K, have headquarters in the state, and Arizona is home to major universities, including the University of Arizona, Arizona State University, and Northern Arizona University. The state is known for a history of conservative politicians such as Barry Goldwater and John McCain, though it has become a swing state since the 1990s.