The highway system of California is a network of roads owned and maintained by the state of California through the California Department of Transportation (Caltrans). Most of these are numbered in a statewide system, and are known as State Route X (abbreviated SR X). United States Numbered Highways are labeled US X, and Interstate Highways are Interstate X, though Caltrans typically uses State Route X for all classes.

Interstate Highways and U.S. Highways are assigned at the national level. Interstate Highways are numbered in a grid—even-numbered routes are east–west routes (with the lowest numbers along Mexico and the Gulf of Mexico), and odd-numbered routes are north–south routes (with the lowest numbers along the Pacific Ocean). U.S. Highways are also numbered in a grid—even numbered for east–west routes (with the lowest numbers along Canada) and odd numbered for north–south routes (with the lowest numbers along the Atlantic Ocean). There are 21 Interstate Highways in California, ranging from Interstate 5 to Interstate 980. There are seven current U.S. Highways including U.S. Route 6 and U.S. Route 395.

California State Routes are managed by Caltrans and designated by the California State Legislature. The state route's signs are in the shape of a miner's spade to honor the California Gold Rush. Each state highway in the U.S. state of California is assigned a Route (officially State Highway Route) number in the Streets and Highways Code (Sections 300-635). Since July 1 of 1964, the majority of legislative route numbers, those defined in the Streets and Highways Code, match the sign route numbers. On the other hand, some short routes are instead signed as parts of other routes — for instance, State Route 112 and State Route 260 are signed as part of the longer State Route 61, and State Route 51 is part of Interstate 80 Business. California County Routes are marked with the usual County route shield, and are assigned a letter for where they are located. For instance, county highways assigned "S" are located in Southern California, ones assigned "J" are found in Central California, and those assigned "A" are located in Northern California.