California Department of Transportation (Caltrans) is a government agency in the U.S. state of California. Its mission is to improve mobility across the state. Caltrans does more than manage the State highway system; it is also actively involved with public transportation systems in California.

For administrative purposes, Caltrans has divided the state of California into 12 districts supervised by district offices. Most districts cover multiple counties; District 12 (Orange County) is the only district with one county. The largest districts are District 4 (San Francisco Bay Area) and District 7 (Los Angeles and Ventura counties). As a state agency, its main headquarters are located in Sacramento, which is covered by District 3.

Caltrans has a reputation for being both innovative and stubbornly idiosyncratic. In 1950, it developed the typeface family, FHWA, now used nationwide on road signs (though not its 2004 replacement, Clearview). It pioneered Botts dots as a superior form of lane marking. It has frequently been criticized for proposing and often constructing ugly bridges and has several times been forced to redesign such structures in response to public outcry. It has long experimented with freeway-to-freeway stack interchanges of increasing height and complexity. It was the last state highway department in the United States to number its freeway exits (the Cal-NExUS program), and one of the last to switch from dark green "button copy" signs to bright green reflective signs. In the 1990s, Caltrans aggressively added carpool lanes on freeways to reduce traffic congestion.

Although state highways generally adhere to consistent minimum design standards throughout much of the state, there are some policy and construction differences between the northern and southern district offices. For example, Northern California carpool lanes are always directly adjacent to mainline traffic lanes and are restricted to carpools only during weekday commute hours, while Southern California carpool lanes are always separated from mainline lanes (except at designated entrance/exit areas) and most are restricted at all times.

Over the last several decades Caltrans has supervised extensive seismic retrofitting of structures throughout California, in addition to creating far more stringent design criteria for new construction.

The earliest predecessor of Caltrans was the Bureau of Highways, which was created by the California Legislature in 1895. This agency consisted of three commissioners who were charged with analyzing the state road system and making recommendations. After the commissioners submitted their report to the Governor on November 25, 1896, the Legislature replaced the Bureau with the Department of Highways.

Due to the state's weak fiscal condition and corrupt politics, little progress was made until 1907, when the Legislature replaced the Department of Highways with the Department of Engineering, within which there was a Division of Highways. The voters approved a $18 million bond issue for the construction of a state highway system in 1910 and the first Highway Commission was convened in 1911. On August 7, 1912, the Department broke ground on its first construction project, the section of El Camino Real between South San Francisco and Burlingame. 1912 also saw the founding of the Transportation Laboratory and the creation of seven administrative divisions (the ancestors of the 12 district offices that exist today).

In 1913, the Legislature started requiring vehicle registration and allocated the resulting funds to support regular highway maintenance, which began the next year.

In 1921, the Legislature turned the Department of Engineering into the Department of Public Works.

In late 1972, the Legislature approved a reorganization (suggested by a study initiated by Governor Ronald Reagan), in which the Department of Public Works was merged with the Department of Aeronautics to become the Department of Transportation.