Caloocan, officially the City of Caloocan (Filipino: Lungsod ng Caloocan; IPA: [kalɔˈʔokan]), is a city in Metro Manila, Philippines. According to the 2024 census, it has a population of 1,712,945 people, making it the fourth-most populous city in the Philippines.

Caloocan is divided into two geographical locations with a total combined area of 5,333.40 hectares (13,179.1 acres), a result of the 1949 expansion of Quezon City, which absorbed much of its territory. It was formerly part of the Province of Rizal in southern Luzon. It comprises what is known as the CAMANAVA area along with the cities of Malabon, Navotas and Valenzuela.

South Caloocan is bordered by Manila, Quezon City, Malabon, Navotas and Valenzuela. The presence of commercial and industrial activities combined with residential areas make it a highly urbanized central business district and a major urban center in the Northern District of Metropolitan Manila. North Caloocan shares its border with Quezon City and Valenzuela, Marilao, Meycauayan and San Jose del Monte in the province of Bulacan, and Rodriguez in the province of Rizal. It is composed of mostly residential subdivisions and extensive resettlement areas with scattered distribution of industrial estates mostly within road transit points and intersections.

Caloocan as a toponym comes from the Tagalog words lo-ok that translates to "bay", sulok that means "corner" suggested by its location between the ends of the present-day Malabon and Tondo, Manila, and kalook-lookan (or kaloob-looban) meaning "innermost area". The city's name is colloquially spelled Kalookan. Historically, it was called Kaloogan, meaning "interior territory," which evolved from the old Tagalog word "loog" (synonymous with "loob," meaning "inside"). The "g" sound could have shifted to the "k" sound in Tagalog phonetics (e.g. baksak → bagsak), leading to the present name of the city.

There is a mixed preference over the preferred spelling of the city's name. Variation, and the apparent confusion over the spelling, came about during the early 1970s, when a resolution was adopted by the municipal board, requiring the city departments to use the name "Kalookan." However, execution of the resolution was interrupted due to martial law imposed nationwide in September 1972. In 1988, Councilor Aurora Asistio-Henson proposed Resolution No. 006, aiming to promote Filipino nationalism by enforcing the spelling "Kalookan" in all city departments and establishments. Despite this, the change was met with opposition from residents, business owners, and officials. Former representative and mayor Virgilio Robles deemed the move illegal, asserting that the city's name is spelled as "Caloocan" according to the city charter. Although confusion persists, the general inclination in the city is towards "Caloocan," as evidenced by the official logo and favored by many barangays, schools, and newspapers. However, some entities, like the Directories of the Philippines Corporation, prefer the spelling "Kalookan."

Originally, Caloocan was the area where the old town of Tondo and Tambobong met, located along the shores of Dagat-Dagatan, a crescent-shaped inland lagoon to the west. The settlement along the shore was called "Aromahan", or "Espina" to the Spaniards, and was separated from Manila Bay by a narrow ridge from Tondo towards an opening in Kinabutasan leading to the sea.

By the late 1700s, the fishermen of Aromahan have expanded towards a hill east of Dagat-Dagatan. This naturally stony hill was called "Kaloogan", meaning "interior territory." With Aromahan relegated to the periphery, this hilltop area was also settled by oppressed people from Tondo, becoming the new center of the community by 1802. To the east was a vast stretch of cogon-covered land. Eventually called "Kalaanan", meaning flat grassland in old Tagalog, this area is now generally known as Grace Park.

Caloocan became a municipality when it was separated from Tondo in 1815. Its original territory extended to the foothills of Marikina, San Mateo and Montalban to the east; Tinajeros, Tanza, and Tala rivers to the north; San Juan del Monte, San Francisco del Monte, Sampalok, Santa Cruz and Tondo in the south; and Dagat-dagatan and Aromahan to the west. The local government building was set up on the relatively well-settled portion just above Libis Espina. The old Aromahan chapel was finally abandoned and a new church was built facing the municipal hall. To escape the Spanish authorities, many from the area abandoned the town proper and sought refuge in the grasslands of Balintawak and Pugad-Lawin, in which the people fought the landlords of Hacienda de Maysilo for terrestrial rights, which went on for almost a hundred years.