Altadena (/ˌɑːltəˈdiːnə/pronunciationⓘAL-tə-DEE-nə) is an unincorporated area,[3][4] and census-designated place in the San Gabriel Valley and the Verdugos[5] regions of Los Angeles County, California.

The name combines the Spanish alta, meaning 'upper,' with dena, a term he likely adapted from Pasadena, though its precise origin remains unclear.

He did the construction for B. D. Wilson and Dr. John Griffin, who jointly owned the Mexican land grant of Rancho San Pascual, about 14,000 acres (5,700 ha), that would be the future sites of these three communities.

They hoped to develop and sell this land in a real estate plan called the San Pasqual Plantation.

Their efforts failed by 1870, despite Eaton's irrigation ditch that drew water from the site of present-day Jet Propulsion Laboratory (JPL) in the Arroyo Seco, because the land was relatively inaccessible and few believed crops could thrive that close to the mountains.

Eaton tried to sell the land for the partners, and in late 1873 he helped broker a deal with Daniel Berry, who represented a group of investors from Indiana, to buy 4,000 acres (1,600 ha) of the rancho.

The land remained primarily agricultural, though several Eastern millionaires built mansions along Mariposa Street, and a small community developed through the 1890s and into the next century.

In 1887 Andrew McNally, the printing magnate from Chicago, and his friend, George Gill Green, had built mansions on what was to become Millionaire's Row: Mariposa Street near Santa Rosa Avenue.

A bit farther east, Zane Grey bought a home from Arthur Herbert Woodward, and added a second-floor study.

The famous Benziger Publishing Company built a mansion on the corner of Santa Rosa Avenue (Christmas Tree Lane) and Mariposa.

He started his career in Altadena with the design and construction of St. Elizabeth of Hungary Catholic Church (parish est.

From 1924 to 1926, 160 homes were built in Altadena by fugitive conman Elisha Paul Janes, with distinctive steep roofs and multiple gables; despite his lack of qualifications, they proved popular, and this neighbourhood was designated as a heritage area in 2002.

[8][9] Many notable buildings followed in the 1930s, including Eliot Junior High School (1931) and Davies Community Center in Farnsworth Park (1934).

In 1993, the Kinneloa Fire, begun accidentally on the slopes above Eaton Canyon, burned dozens of homes in Altadena and neighboring Kinneloa Mesa as part of a rash of late October wildfires driven by Santa Ana winds in Southern California.

[14][15] In 2022, Altadena gained local coverage in Los Angeles as the place of the first land return to the Tongva since the arrival of Europeans in the Los Angeles Basin area, after a resident donated her 1 acre property to the Tongva Taraxat Paxaavxa Conservancy.

[18] In 2022, a single lottery ticket was sold to Edwin Castro, which would win a world record US$2.04 billion Powerball jackpot.

[19] On January 7, 2025, the Eaton Fire started in Altadena around 6:30 PM local time during a powerful Santa Ana windstorm.

[21] For statistical purposes, the United States Census Bureau has defined Altadena as a census-designated place (CDP).

According to the 2010 United States Census, Altadena had a median household income of $82,895, with 10.7% of the population living below the federal poverty line.

[48][49][50] Christmas Tree Lane is a 0.7-mile (1.1 km) stretch of Santa Rosa Avenue from Woodbury Road to Altadena Drive.

Chaney Trail, just west of the intersection at Fair Oaks Avenue and Loma Alta Street, is a forestry service road leading to the old right of way.

The Cobb Estate at the top of Lake Avenue is now a free botanical garden, operated by the United States Forest Service.

It is guarded by its historic gates, which are easily bypassed to allow visitors and hikers to ascend its long and winding paved driveway to the site of what was once one of Altadena's premier mansions.

This site is also found alongside the Sam Merrill Trail, which accesses Las Flores Canyon on the way to Echo Mountain.

Crudely Hewn Tombstone is the final resting place of abolitionist and Harpers Ferry attack survivor Owen Brown.