The 25-year power purchase agreement (PPA) will see Apple receive 130MW of energy.
Pacific Gas & Electric will buy the remaining 150MW output of the project under a separate PPA.
First Solar chief commercial officer Joe Kishkill said: "Apple’s commitment was instrumental in making this project possible and will significantly increase the supply of solar power in California.
"Over time, the renewable energy from California Flats will provide cost savings over alternative sources of energy as well as substantially lower environmental impact."
The project is located on 2,900 acres of private land in southeast Monterey County, near the San Luis Obispo, Kings and Fresno County borders.
Upon completion, the project will generate enough solar energy for around 100,000 average homes per year and reduce more than 109,000 tons of carbon dioxide emissions.
The project will also displace more than 152,000 tons of water consumption per year based on the average California grid.
Construction on the plant is anticipated to start in mid-2015 and complete by the end of 2016.
Earlier in 2015, the Monterey County Planning Commission approved the California Flats solar project. It still needs approval from the Monterey County Board of Supervisors.
