Rancho Trabuco was a 22,184-acre (89.78 km2) Mexican land grant in present-day Orange County, California. The five square league grant consisted of two square leagues given in 1841 by Governor Juan B. Alvarado to Santiago Argüello plus three square leagues given in 1846 by Governor Pío Pico to John (Don Juan) Forster. The name "trabuco" means "blunderbuss" in Spanish. The grant extended along Trabuco Creek and Trabuco Canyon, and encompassed present-day Coto de Caza. The grant was bordered by Rancho Cañada de los Alisos on the west, and by Rancho Mission Viejo on the east.
Santiago Argüello was a soldier in the Mexican army, and was commandant at the Presidio of San Diego from 1830 to 1835. In 1841, Argüello was granted the two square league Rancho Trabuco for his services. In 1846 Argüello was granted Rancho Ex-Mission San Diego. John Forster, who was the grantee of the adjacent Rancho Mission Viejo in 1845, acquired Argüello's Rancho Trabuco and was granted an additional three square leagues in 1846.
With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored. As required by the Land Act of 1851, a claim for Rancho Trabuco was filed with the Public Land Commission in 1852,  and the grant was patented to John Forster in 1866.
In 1864, Forster added Rancho Santa Margarita y Las Flores to his holdings. Forster died in 1882.
Irish immigrants, Richard O'Neill Sr. and James Flood, acquired the rancho in 1882. Flood and O'Neill became equal partners of the Rancho Santa Margarita y las Flores, Rancho Mission Viejo and Rancho Trabuco lands. In 1907, James L. Flood, son of the original owner, made good on his late father's promise and conveyed an undivided half interest to O'Neill, Sr. Four months later, declining health caused O'Neill to deed his interest to his son, Jerome. In 1923, the sons of Flood and O'Neill consolidated their partnership with the Santa Margarita Company. In 1939, property was divided. Richard O'Neill Jr. retained the Rancho Mission Viejo and Rancho Trabuco portion located in Orange County, and the Flood family took the Rancho Santa Margarita y las Flores property in San Diego County. Most of Rancho Trabuco is now the site of O'Neill Regional Park
- Ogden Hoffman, 1862, Reports of Land Cases Determined in the United States District Court for the Northern District of California, Numa Hubert, San Francisco
- Diseño del Rancho Trabuco
- Spanish and Mexican Ranchos of Orange County
- History of San Diego
- United States. District Court (California : Southern District) Land Case 216 SD
- Report of the Surveyor General 1844 - 1886
- "O'Neill Regional Park History". OC Parks. www.ocparks.com. Retrieved 2010-05-24.