Callao (Spanish pronunciation: [kaˈʎao]) is a seaside city and constitutional province in Peru. Located next to the Pacific Ocean, it forms part of the Lima metropolitan area alongside Lima province. Its autonomous status dates back to 1836, when it was declared a Littoral Province of the Peru–Bolivian Confederation, after which it was declared a Constitutional Province by the government of Ramón Castilla in 1857.

Callao is Peru's chief seaport and home to its main airport, Jorge Chávez International Airport. Founded in 1537 by the Spaniards, the city has a long naval history as one of the main ports in Latin America and the Pacific, having been a vital Spanish town during the colonial era. Central Callao is about 15 km (9.3 mi) west of the Historic Centre of Lima.

The oldest traces of human occupation of Callao date back more than 10,000 years and are the stone workshops on Chivateros Hill. Throughout the pre-Inca period, various peoples settled in the area, primarily dedicated to fishing. In the mid-15th century, the territory of Callao was annexed to the Inca Empire, with the Inca Palace of Oquendo being one of its strategic centres.

The city was founded by Spanish colonists in 1537, just two years after Lima, which was founded in 1535. The origin of its name is unknown; both Amerindian (particularly Yunga, or Coastal Peruvian) and Spanish sources are credited, but it is certain that it was known by that name since 1550. Other sources point to the similarity with the Portuguese word calhau [pebble], having a similar sound.

It soon became the main port for Spanish commerce in the Pacific. At the height of the Viceroyalty, virtually all goods produced in Peru, Bolivia, and Argentina were carried over the Andes by mule to Callao, to be shipped to Panama, carried overland, and then transported on to Spain via Cuba. The port of Callao was also a node in the Manila galleon route connecting Latin-America and Asia through Acapulco, Mexico and Manila, Philippines. As a result, Callao also became a permanent target for pirate and corsair attacks, such as the one carried out by Francis Drake in 1579 and the blockade established by Jacques l'Hermite in 1624.

Callao took great importance during the Peruvian War of Independence. After the Battle of Ayacucho, 9 December 1824, that sealed the independence of Peru and South America, Spain made futile attempts to retain its former colonies, such as at the second siege of Callao.

In a session on March 8, 1834, the National Convention presided over by Francisco Xavier de Luna Pizarro decorated Callao as “The Faithful and Generous City of Callao, Asylum of the Laws and Liberty”, as a reward for defending the government of General Luis José de Orbegoso against the coup attempts of General Pedro Pablo Bermúdez.

On 20 August 1836, during the Peru–Bolivian Confederation, President Andrés de Santa Cruz mandated the creation of Callao Littoral Province (Spanish: Provincia Litoral del Callao), also called the Military and Political Government of Callao (Spanish: Gobierno Político y Militar del Callao), which had political autonomy in its internal affairs. The province was thus separated from Lima from this point onwards.