Baja California peninsula in the context of "Mazatlán"

The Gulf of California (Spanish: Golfo de California), also known as the Sea of Cortés (Mar de Cortés) or Sea of Cortez, or less commonly as the Vermilion Sea (Mar Vermejo), is a marginal sea of the Pacific Ocean that separates the Baja California peninsula from the Mexican mainland. It is bordered by the states of Baja California, Baja California Sur, Sonora, and Sinaloa with a coastline of approximately 4,000 km (2,500 mi). Rivers that flow into the Gulf of California include the Colorado, Fuerte, Mayo, Sinaloa, Sonora, and the Yaqui. The surface of the gulf is about 160,000 km (62,000 sq mi). Maximum depths exceed 3,000 meters (9,800 ft) because of the complex geology, linked to plate tectonics.

The gulf is thought to be one of the most diverse seas on Earth and is home to more than 5,000 species of micro-invertebrates. Parts of the Gulf of California are a UNESCO World Heritage Site.

Baja California, officially the Free and Sovereign State of Baja California, is a state in Mexico. It is the northwesternmost of the 32 federal entities of Mexico. Before becoming a state in 1952, the area was known as the North Territory of Baja California (Territorio Norte de Baja California). It has an area of 70,113 km (27,071 sq mi) (3.57% of the land mass of Mexico) and comprises the northern half of the Baja California peninsula, north of the 28th parallel, plus oceanic Guadalupe Island. The mainland portion of the state is bordered on the west by the Pacific Ocean, on the east by Sonora and the northern Gulf of California, on the north by the United States and on the south by Baja California Sur.

The state has an estimated population of 3,769,020 as of 2020, significantly higher than the sparsely populated Baja California Sur to the south, and similar to San Diego County, California, to its north. Over 75% of the population lives in Mexicali (the state's capital city), Ensenada, or Tijuana (the state's largest city). Other important cities include San Felipe, Rosarito, and Tecate.

Alta California (English: Upper California), also known as Nueva California (English: New California) among other names, was a province of New Spain formally established in 1804. Along with the Baja California peninsula, it had previously comprised the province of Las Californias, but was made a separate province in 1804 (named Nueva California). Following the Mexican War of Independence, it became a territory of Mexico in April 1822 and was renamed Alta California in 1824.

The territory included all of the present-day U.S. states of California, Nevada, and Utah, and parts of Arizona, Wyoming, and Colorado. The territory was re-combined with Baja California (as a single departamento) in Mexico's 1836 Siete Leyes (Seven Laws) constitutional reform, granting it more autonomy. That change was undone in 1846, but rendered moot by the outcome of the Mexican–American War in 1848, when most of the areas formerly comprising Alta California were ceded to the U.S. in the treaty which ended the war. In 1850, California joined the union as the 31st state.

The Peninsular Ranges (also called the Lower California province) are a group of mountain ranges that stretch 930 mi (1,500 km) from Southern California to the southern tip of the Baja California peninsula; they are part of the North American Pacific Coast Ranges, which run along the Pacific coast from Alaska to Mexico. Elevations range from 500 to 10,834 ft (150 to 3,300 m).

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.

Guadalupe Island (Spanish: Isla Guadalupe) is a volcanic island located 241 kilometres (130 nautical miles; 150 miles) off the western coast of Mexico's Baja California peninsula and about 400 km (220 nmi; 250 mi) southwest of the city of Ensenada in the state of Baja California, in the Pacific Ocean. The various volcanoes are extinct or dormant. In 2005 Guadalupe Island and its surrounding waters and islets were declared a biosphere reserve to restore its vegetation (decimated by feral goats) and to protect its population of marine mammals and birds. The island was a popular destination for great white shark cage diving before a tourism ban was put in place in 2022. Guadalupe Island is inhabited only by scientists, military personnel operating a weather station, and a small group of seasonal fishermen. The island is mostly arid and has very little surface water.

The two other Mexican island groups in the Pacific Ocean that are not on the continental shelf are the Revillagigedo Islands and Rocas Alijos. Guadalupe Island and its islets are the westernmost region of Mexico.