Baja California Sur, officially the Free and Sovereign State of Baja California Sur, is a state in Mexico. It is the 31st and last state to be admitted, in 1974. It is also the least densely populated Mexican state, the second least populous, and the ninth-largest state by area.

Before becoming a state on 8 October 1974, the area was known as the El Territorio Sur de Baja California ("South Territory of Lower California"). It has an area of 73,909 km² (28,536 sq mi), or 3.57% of the land mass of Mexico, and occupies the southern half of the Baja California Peninsula, south of the 28th parallel, plus the uninhabited Rocas Alijos in the Pacific Ocean. It is bordered to the north by the state of Baja California, to the west by the Pacific Ocean, and to the east by the Gulf of California. The state has maritime borders with Sonora and Sinaloa to the east, across the Gulf of California.

The state is home to the tourist resorts of Cabo San Lucas and San José del Cabo. Its largest city and capital is La Paz.

The first European known to have landed in Baja California was a rebel navigator named Fortún Ximénez, who killed his captain and one of the envoys of Hernán Cortés, Diego de Becerra [es] in his sleep, while he led a revolt and ruined the entire mission. Ximénez and the accomplice crewmen still in navigation ruined the sail in the peninsula and were hostile with the Indians due to their collecting giant pearls and because explorers wanted to rape the women. This started a brawl with the Indians and Ximénez was murdered. The surviving Spanish navigators withdrew to the coast and sailed erratically for several days until they reached the shores of the present-day Jalisco, where they encountered a subaltern of Nuño de Guzmán, who personally did not like Cortés, and requisitioned the ship and took them prisoner reporting mistakenly that the peninsula was an island and starting its initial European misconception as an island. After three attempts by Hernán Cortés to colonize the area for Spain, included one mission commanded by himself, someone who also personally did not like Cortés and a writer of the time cited as "Alarcón", started calling the region as "California", a mythical island described in the popular novel of the time Las Sergas de Esplandián as a mock against Cortés, creating the tradition of name as it the region. The name California applied to this peninsula along with the areas now known as the state of California and part of the state of Arizona in the United States.

In 1888, in order to manage the mining in the region, the federal government under Porfirio Diaz divided the peninsula in north and south, creating a federal territory named El Territorio Sur de Baja California (Spanish: South Territory of Lower California) and the territory became a state on 8 October 1974, named after the peninsula on which it is found, Baja (Lower) California, with the term "Sur" meaning "south".

Japanese archaeologist Harumi Fujita, who has been excavating the Cape Region since 1985, has carbon-dated remains from the Babisuri Shelter on the Isla Espíritu Santo to 40,000 years ago, placing the earliest habitation date in the Archaic period, though the majority of remains indicate indigenous people have constantly occupied the area from between 10,000 and 21,000 years ago. Evidence of early human habitation is found in primitive rock and cave paintings dating to 1700 BCE, created by hunting and gathering societies that lived in rock shelters. The state is one of five areas in the world with important concentrations of cave paintings. These paintings have an identifiable style and tend to be on a monumental scale with some figures as tall as 4 metres (13 ft). Most of the animals are painted in silhouette and depicted in movement, often being hunted by people. The best known site is the Great Mural Rock Art, which dates from 1700 BCE, located in the north of the state. Other important sites include Cueva de Palma, San Gregorio, Santa Teresa, Guadalupe, San Francisco, Cabo Pulmo, Santiago and San Borjita. The most important concentrations are in twelve square kilometer zone in the north of the state, centered on the Sierra de San Francisco. In sites near Comondú, Las Palmas and Cocheros, there are also arrowheads, utensils and petroglyphs. Las Palmas contains secondary burials of human bones painted with red ochre.

When the Spanish arrived, there were four main ethnic groups: the Pericúes in the south between Cabo San Lucas and La Paz, the Guaycuras in the area north of the Pericú to Loreto, the Monquis near Loreto and Cochimí in the middle of the peninsula. All were hunter/gatherers without agriculture or metalworking, but produced pottery. They also fished, but only the Pericúes had rafts.

The first Spaniard in the area is believed to be Fortún Ximénez, arriving in 1533. He and his crew did not remain long because they sacked the area's pearls and abused the women, prompting a violent confrontation with the natives, who killed Ximénez. The remaining crew returned to Mexico City with the pearls and stories of riches.