Tucson (/ˈtuːsɒn/; O'odham: Cuk Ṣon; Spanish: Tucsón) is the county seat and most populated city of Pima County, Arizona, United States. It is the second-most populous city in Arizona with a population of 542,630 at the 2020 census, behind only Phoenix, while the Tucson metropolitan statistical area has an estimated 1.08 million residents and is the 52nd-largest metropolitan area in the U.S. Tucson and Phoenix anchor the Arizona Sun Corridor. The city is 108 miles (174 km) southeast of Phoenix and 60 mi (100 km) north of the United States–Mexico border. It is home to the University of Arizona.

Major incorporated suburbs of Tucson include Oro Valley and Marana northwest of the city, Sahuarita south of the city, and South Tucson in an enclave south of downtown. Communities in the vicinity of Tucson (some within or overlapping the city limits) include Casas Adobes, Catalina Foothills, Flowing Wells, Midvale Park, Tanque Verde, Tortolita, and Vail. Towns outside the Tucson metropolitan area include Three Points, Benson to the southeast, Catalina and Oracle to the north, and Green Valley to the south.

Tucson was founded as a military fort by the Spanish when Hugo O'Conor authorized the construction of Presidio San Agustín del Tucsón in 1775. It was included in the state of Sonora after Mexico gained independence from the Spanish Empire in 1821. The United States acquired a 29,670 square miles (76,840 km²) region of present-day southern Arizona and southwestern New Mexico from Mexico under the Gadsden Purchase in 1853. Tucson served as the capital of the Arizona Territory from 1867 to 1877. Tucson was Arizona's largest city by population during the territorial period and early statehood, until it was surpassed by Phoenix by 1920. Nevertheless, its population growth remained strong during the late 20th century. Tucson was the first American city to be designated a "City of Gastronomy" by UNESCO in 2015.

The Spanish name of the city, Tucsón (Spanish pronunciation: [tuɣˈson]), is derived from the O'odham Cuk Ṣon (O'odham pronunciation: [tʃʊk ʂɔːn]). Cuk is a stative verb meaning "(be) black, (be) dark". Ṣon is (in this usage) a noun referring to the base or foundation of something. The name is commonly translated into English as "the base [of the hill] is black", a reference to a basalt-covered hill now known as Sentinel Peak. Tucson is sometimes referred to as the Old Pueblo and Optics Valley, the latter referring to its optical science and telescopes known worldwide.

Spanish Empire 1775–1821
First Mexican Empire 1821–1823
United Mexican States 1823–1854
 United States 1854–present

The Tucson area was probably first visited by Paleo-Indians, who were known to have been in southern Arizona about 12,000 years ago. Recent archaeological excavations near the Santa Cruz River found a village site dating from 2100 BC. The floodplain of the Santa Cruz River was extensively farmed during the Early Agricultural Period, c. 1200 BC to AD 150. These people hunted, gathered wild plants and nuts, and ate corn, beans, and other crops grown using irrigation canals they constructed.

The Early Ceramic period occupation of Tucson had the first extensive use of pottery vessels for cooking and storage. The groups designated as the Hohokam lived in the area from AD 600 to 1450 and are known for their vast irrigation canal systems and their red-on-brown pottery.

Italian Jesuit missionary Eusebio Francisco Kino first visited the Santa Cruz River valley in 1692. He founded the Mission San Xavier del Bac in 1700, about 7 mi (11 km) upstream from the site of the settlement of Tucson. A separate Convento settlement was founded downstream along the Santa Cruz River, near the base of what is now known as "A" mountain. Hugo Oconór (Hugo O'Conor), the founding father of the city of Tucson, Arizona, authorized the construction of a military fort in that location, Presidio San Agustín del Tucsón, on August 20, 1775 (the present downtown Pima County Courthouse was built near this site). During the Spanish period of the presidio, attacks such as the Second Battle of Tucson were repeatedly mounted by the Apache. Eventually, the town came to be called Tucsón, a Spanish version of the O'odham word for the area. It was included in the state of Sonora after Mexico gained independence from the Kingdom of Spain and its Spanish Empire in 1821