Santiago Peak is the southern mountain of the Saddleback landform in Orange County and Riverside County, California. It is the highest and most prominent peak of both the Santa Ana Mountains and Orange County. The peak is named for Santiago Creek, which begins on its southwestern slope. During most winters, snow falls at least once on the peak. A telecommunication antenna farm with microwave antennas sits atop the peak.

The Acjachemen referred to the peak as Kalawpa, with the nearby village of Alume meaning "to raise the head in looking upward," in reference to the mountain.

More than one trail leads to the top of Santiago Peak, but the most popular among hikers is the Holy Jim trail. The Holy Jim trail gains about 4,000 feet (1,200 m) in elevation and is a 16-mile (26 km) round trip. It is a moderate to strenuous hike and is most enjoyable during spring and winter due to the large number of insects during warmer times of the year.

From the summit of Santiago Peak, one can see the larger Southern California peaks like San Gorgonio Mountain, San Jacinto Peak, and Mount San Antonio. However, due to the large number of antennas at the top of Santiago Peak, a full 360-degree view of the surrounding landscapes is not possible. Those at the top must walk approximately a quarter-mile around the perimeter of all the antennas to take in views of every direction.

Santiago Peak is a radio site with buildings owned by American Tower, Crown Castle, MobileRelay Associates, Day Wireless, Orange County Communications, the United States Federal Government, the State of California, and Southern California Edison, among others.

Santiago Peak provides radio coverage over much of Los Angeles, Orange, San Bernardino, Riverside, and San Diego counties. It houses both broadcast and two-way communications facilities on virtually every frequency band, including FM broadcast, VHF low- and high-band, UHF, 800/900 MHz, and microwave.

In September 2024, the Airport Fire, which originated in Trabuco Canyon, burned to Santiago Peak and damaged parts of the radio infrastructure.

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  Santiago Peak looking north (November 2021)
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  Santiago Peak at sunset, looking west (November 2021)
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  Center of Santiago Peak, looking north (November 2021)
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  Santiago Peak at sunset, looking southwest towards San Clemente (November 2021)

• List of highest points in California by county

• "Holy Jim Trail - Santa Ana Mountains - California". Peak Fever.