The Californias (Spanish: Las Californias), occasionally known as the Three Californias or the Two Californias, are a region of North America spanning the United States and Mexico, consisting of the U.S. state of California and the Mexican states of Baja California and Baja California Sur. Historically, the term Las Californias was used to define the vast northwestern region of Spanish America, as the Province of the Californias (Spanish: Provincia de las Californias), and later as a collective term for Alta California and the Baja California peninsula.

Originally a single, vast entity within the Spanish Empire, administration was split into Baja California (Lower California) and Alta California (Upper California) following the Mexican War of Independence. As a part of the Mexican–American War (1846–48), the Conquest of California saw the vast Alta California territory ceded from Mexico to the United States. The populated coastal region of the territory was admitted into the Union in 1850 as the State of California, while the vast, sparsely populated interior region would only later gain statehood as Nevada, Utah, and parts of New Mexico, Arizona, Wyoming, and Colorado.

Today, "the Californias" is a collective term to refer to the American and Mexican states bearing the name California, which share geography, history, cultures, and strong economic ties. The Commission of the Californias is a tri-lateral forum for cooperation between the three Californian state governments.

There has been understandable confusion about use of the plural Californias by Spanish colonial authorities. California historian Theodore Hittell offered the following explanation:

In very early times, while the country was supposed to be an island or rather several islands, it was commonly known by the plural appellation of "Las Californias" (The Californias). Afterwards, when its peninsular character was ascertained, it was called simply California; but the territory so designated was unlimited in extent. When the expeditions for the settlement of San Diego and Monterey marched, it was understood that they were going, not out of California, but into a new part of it. The peninsula then began to be generally spoken of as Antigua or Old California and the unlimited remainder as Nueva or New California, subsequently more commonly called Alta or Upper California. At the same time the old plural name of The Californias was revived, but with a more definite signification than before.

The first attempted Spanish occupation of California was by the Jesuit missionary Eusebio Kino, in 1683. His Misión San Bruno failed, however, and it was not until 1697 that Misión de Nuestra Señora de Loreto Conchó was successfully established by another Jesuit, Juan María de Salvatierra. The mission became the nucleus of Loreto, first permanent settlement and first administrative center of the province. The Jesuits went on to found a total of 18 missions in the lower two-thirds of the Baja California Peninsula.

In 1767, the Jesuits were expelled from the missions, and Franciscans were brought in to take over. Gaspar de Portolá was appointed governor to supervise the transition. At the same time, a new visitador, José de Gálvez, was dispatched from Spain with authority to organize and expand the fledgling province.

The more ambitious province name, Las Californias, was established by a joint dispatch to the King from Viceroy de Croix and visitador José de Gálvez, dated January 28, 1768. Gálvez sought to make a distinction between the Antigua ('old') area of established settlement and the Nueva ('new') unexplored areas to the north. At that time, almost the only explored and settled areas of the province were around the former Jesuit missions but, once exploration and settlement of the northern frontier began in earnest, the geographical designations Alta ('upper') and Baja ('lower') gained favor.