Portola Valley is a small town in San Mateo County, California, United States. As of the 2020 census, the town had a population of 4,456. Located on the San Francisco Peninsula in the Bay Area, the community is nestled on the eastern slopes of the Santa Cruz Mountains.

Portola Valley was named for Spanish explorer Gaspar de Portolá, who led the first party of Europeans to explore the San Francisco Peninsula in 1769.

The Native Americans already present were Ohlone and specifically the group (or groups) known as Olpen or Guemelento but these were later moved to Mission Dolores and Mission Santa Clara de Asís which claimed the land and peoples. The area's written history dates back to 1833, when a square league of land was given to Domingo Peralta and Máximo Martínez by Governor José Figueroa to form the Rancho Cañada del Corte de Madera. In those days it was used for lumbering and cattle grazing.

By the 1880s Andrew S. Hallidie, a wire rope manufacturer, had built his country home of Eagle Home Farm in what is now Portola Valley. He built a 7,341 foot long aerial tramway from his house to the top of Skyline in 1894 though it was removed after his death in 1900.

In 1886 the name Portola-Crespi Valley was bestowed on the area from the then community of Crystal Springs (now under Crystal Springs Reservoir to the then community of Searsville (in the area of the present day Jasper Ridge Biological Preserve); Crespi is for Juan Crespí, a Franciscan friar with the Portolà expedition.

The town was incorporated in 1964. Bill Lane, known as the publisher of Sunset magazine, was its first mayor.

The conversion of Portola Valley from large ranches to residences began in earnest after World War II, when housing pressures from the growing technology industry expanded out from Santa Clara County to reach San Mateo County.

For Portola Valley, this began with the sub-division of the Ormondale Ranch to form the Westridge neighborhood and Home Owners' Association in 1947.