Tijuana (US: tee-HWAH-nə, Spanish: [tiˈxwana] (listen)) is a city in Baja California, Mexico. It is part of the San Diego–Tijuana transborder urban agglomeration and the larger Southern California megalopolis. As the second-largest city in Mexico and center of the 6th-largest metro area in Mexico, Tijuana exerts a strong influence in education and politics, in transportation, culture, art, and manufacturing and as a migration hub. Currently one of the fastest-growing metropolitan areas in Mexico, Tijuana is rated as a "High Sufficiency" global city by the Globalization and World Cities Research Network. As of September 2019, the city of Tijuana had a population of 1,810,645, with its metropolitan area containing a population of 2,157,853 as of 2020, an estimated 2,002,000 within the urban area.Tijuana is located on the Pacific coast of Baja California, and is the municipal seat and the cultural and commercial center of Tijuana Municipality. Tijuana covers 70% of the territory of the municipality and contains 80% of its population. A dominant manufacturing center of the North American continent, the city maintains facilities of many multinational conglomerate companies. In the early 21st century, Tijuana became the medical-device manufacturing capital of North America. Tijuana is also a growing cultural center and has been recognized as an important new cultural mecca. The city is the most visited border city in the world; sharing a border of about 24 km (15 mi) with its sister city San Diego. More than fifty million people cross the border between these two cities every year. This metropolitan crossing makes the San Ysidro Port of Entry the fourth busiest land-border crossing in the world. It is estimated that the two border crossing stations between the cities proper of San Diego and Tijuana account for 300,000 daily border crossings alone.Tijuana is the 45th largest city in the Americas and is the westernmost city in Mexico. According to the 2015 census, the Tijuana metropolitan area was the fifth-largest in Mexico, with a population of 1,840,710, but rankings vary, the city (locality) itself was 6th largest and the municipality (administrative) 3rd largest nationally. The international metropolitan region was estimated at 5,158,459 in 2016, making it the third-largest metropolitan area in the former Californias region, 19th largest metropolitan area in the Americas, and the largest bi-national conurbation that is shared between US and Mexico. Tijuana is becoming more suburbanized like San Diego.

Tijuana traces its modern history to the arrival of Spanish explorers in the 16th century who were mapping the coast of the Californias. As the American occupation of the Mexican capital ended with the Treaty of Guadalupe Hidalgo, Tijuana's new international position on the border gave rise to a new economic and political structure. The city was founded on July 11, 1889 as urban development began. Often known by its supposed initials, T.J., and nicknamed Gateway to Mexico, the city has historically served as a tourist center dating back to the 1880s.