California State Route 99 is a 415 mile (668 km) highway that traverses California's Central Valley from the north at Red Bluff to the south near Bakersfield. The majority of its length is built to freeway standards, though it is at times a two-lane rural highway or a divided highway.

Route 99 was originally part of U.S. Highway 99 which was decommissioned in 1968 after the completion of Interstate 5. Since the remnant did not cross state lines, it was not allowed to keep its federal highway status. Many of the older highway signs in the southbound lanes still display a control city of Los Angeles, even though CA-99 no longer runs through that city. From the north, Route 99 runs generally parallel to, and to the east of Interstate 5. It begins by splitting off from I-5 at Red Bluff, and serves as a two-lane highway, running through Chico and Yuba City to the state capital, Sacramento. There, it is promoted to a freeway and meets I-5 briefly before diverging from it again. The two freeways run somewhat close to one another for about 90 miles, but after passing through Stockton, they begin to diverge more and more.

The freeway portions connect and serve the numerous small cities that mostly support the agriculture and industry of the Central Valley and these segments provide a fast medium distance haulage route connecting agricultural production with related processing and packing businesses. Traveling southbound form Stockton, Route 99 passes through the cities of the San Joaquin Valley, while I-5 is relegated to less densely populated areas. Travelers on their way to Los Angeles from Stockton and Sacramento, or from Los Angeles to the Bay Area would be well advised to take I-5 and avoid the traffic jams of Route 99. Route 99 continues through Modesto, Merced, Fresno, Visalia, Tulare and Bakersfield. A few miles before the Tehachapi Mountains and Grapevine Hill, the road made famous by Commander Cody and The Lost Planet Airmen in their song, "Hot Rod Lincoln ," Route 99 meets up with I-5 again and comes to an end in Wheeler Ridge.

Recently, it has been recommended that Route 99 be upgraded to interstate highway standards between Red Bluff and Stockton (or Sacramento), which would require upgrading some substandard sections and eliminating the last at-grade intersections. Caltrans has recommended Interstate 9 as the designation of the route.