San Diego–Tijuana is an international transborder agglomeration, straddling the border of the adjacent North American coastal cities of San Diego, California, United States, and Tijuana, Baja California, Mexico. The 2020 population of the region was 5,456,577, making it the largest bi-national conurbation shared between the United States and Mexico, and the second-largest shared between the US and another country. The conurbation consists of the San Diego metropolitan area, in the United States and the municipalities of Tijuana, Rosarito Beach (126,980), and Tecate (108,440) in Mexico. It is the third most populous region in the California–Baja California region, smaller only than the metropolitan areas of Greater Los Angeles and the San Francisco Bay Area.

A transborder agglomeration is an urban agglomeration or conurbation that extends into multiple sovereign states and/or dependent territories. It includes city-states that agglomerate with their neighbouring countries.