Hernando de Alarcón (born c. 1500) was a Spanish explorer and navigator of the 16th century, noted for having led an early expedition to the Baja California Peninsula, during which he became one of the first Europeans to ascend the Colorado River from its mouth and perhaps the first to reach Alta California. 
Little is known about Alarcón's life outside of his exploits in New Spain. He was probably born in the town of Trujillo, in present-day Extremadura, Spain, in the first years of the 16th century and traveled to the Spanish colonies in the Americas as a young man. 
By 1540, Mexico had been conquered and state-sponsored expeditions were being sent north in search of new wealth and the existence of a water passage between the Atlantic and Pacific oceans. Viceroy of New Spain Antonio de Mendoza commissioned Francisco Vázquez de Coronado to undertake a massive overland expedition with the purpose of finding the Seven Cities of Cibola, which were rumored to exist in the unexplored northern interior. The expedition was to be resupplied with stores and provisions delivered by ships traveling up the Sea of Cortés, the commander of which would be Alarcón.
Alarcón set sail from Acapulco with two ships, the San Pedro and the Santa Catalina, on May 9, 1540, and was later joined by the San Gabriel at St. Jago de Buena Esperanza, in Colima.  His orders from Mendoza were to await the arrival of Coronado's land expedition at a certain latitude along the coast. The meeting with Coronado was never effected, though Alarcón reached the appointed place and left letters, which were soon afterwards discovered by Melchior Diaz, another explorer.
Alarcón eventually sailed to the northern terminus of the Gulf of California and completed the explorations begun by Francisco de Ulloa the preceding year. During this voyage Alarcón proved to his satisfaction that no open-water passage existed between the Gulf of California and the South Sea. Subsequently, on September 26, he entered the mouth of the Colorado River, which he named the Buena Guia. He was the first European to ascend the river for a distance considerable enough to make important observations. On a second voyage, he probably proceeded past the present-day site of Yuma, Arizona. A map drawn by one of Alarcón's pilots is the earliest accurately detailed representation of the Gulf of California and the lower course of the Colorado River.
Alarcón is almost unique among 16th-century conquistadores in that he reportedly treated the Indians he met humanely, as opposed to the often reckless and cruel behavior known from accounts of his contemporaries. Bernard de Voto, in his 1953 Westward the Course of Empire, observed: "The Indians had an experience they were never to repeat: they were sorry to see these white men leave." Alarcón wrote of his contact with the Yuma-speaking Indians along the Colorado. The information he compiled consisted of their practices in warfare, religion, curing and even sexual customs.
California Historical Landmark No. 568, on the west bank of the Colorado River near Andrade in Imperial County, California, commemorates Alarcón's expedition having been the first non-Indians to sight land within the present-day state of California.  
- Elsasser, Albert B. (1979). "Explorations of Hernando Alarcón in the Lower Colorado River Region, 1540". Journal of California and Great Basin Anthropology. 1 (1).
- "Hernando de Alarcon". Social Studies Fact Cards: California Explorers. Toucan Valley Publications, Inc. Retrieved 3 May 2017.
- Burney, James (2010). A Chronological History of the Discoveries in the South Sea or Pacific Ocean (Digitally printed ed.). New York: Cambridge University Press. ISBN 978-1-108-02408-2.
- "CHL No. 568 Hernando de Alarcón Expedition - Imperial". California Historical Landmarks. CaliforniaHistoricalLandmarks.com. Retrieved 5 May 2017.
- Detling, Douglas G. (2012). "California Discovered" (PDF). Detling Family News. Retrieved 5 May 2017.
- Hammond, George P. & al., ed. Narratives of the Coronado Expedition, 1540-1542. University of New Mexico Press: Albuquerque, New Mexico. 1940.