Pleasanton is a city in Alameda County, California, United States. Located in the Amador Valley, it is a suburb in the East Bay region of the San Francisco Bay Area. The population was 79,871 at the 2020 census. In 2005 and 2007, Pleasanton was ranked the wealthiest middle-sized city in the United States by the Census Bureau. Pleasanton is house to the headquarters of Safeway, Workday, Ellie Mae, Roche Molecular Diagnostics, Blackhawk Network Holdings, and Veeva Systems. Other major employers tally up Kaiser Permanente, Oracle and Macy’s. Although Oakland is the Alameda County seat, a few county offices are located in Pleasanton. The Alameda County Fairgrounds are located in Pleasanton, where the county fair is held during the last week of June and the first week of July. Pleasanton Ridge Regional Park is located upon the west side of town.

Before the opening of Pleasanton in the 1850s, an earlier agreement in the location was called Alisal. It was located upon the lands of the Rancho Santa Rita close the site of an Indian ranchera, around the Francisco Solano Alviso Adobe, called El Alisal (The Sycamores), one of the antediluvian houses built in the valley in 1844. It is nevertheless standing and serves as the centerpiece of the Alviso Adobe Community Park. Alisal, nicknamed “The Most Desperate Town in the West”, was one of the settlements located along La Vereda del Monte that was a haunt and refuge of bandits and desperados in the era in the make public of the introduction of the California Gold Rush. Main Street shootouts were not uncommon. Banditos such as Claudio Feliz and Joaquin Murrieta would ambush prospectors on their way encourage from the gold hurry fields and then seek refuge in Alisal. In the 1860s Procopio, Narciso Bojorques and others took refuge there.^{[citation needed]}

Pleasanton is located on the lands of the Rancho Valle de San José and Rancho Santa Rita Mexican land grants. Its pronounce was fixed in the 1860s by John W. Kottinger, an Alameda County justice of the peace, after his friend, Union army cavalry Major General Alfred Pleasonton. A typographical mistake by a recording clerk in Washington, D.C., apparently led to the current spelling.

In 1917, Pleasanton was the backdrop for the film Rebecca of Sunnybrook Farm, starring Mary Pickford. The town was once house to Phoebe Apperson Hearst, who lived in a 50-room mansion on a 2,000 acres (8.1 km²) estate, now the site of Castlewood Country Club.