U.S. Route 399 was a U.S. Highway in the state of California that ran from Ventura to Bakersfield.

Leaving Ojai, it continued into the Los Padres National Forest along the Maricopa Highway, with its summit at Pine Mountain.

Descending into the Cuyama River Valley, it met SR 166 and travelled east towards Maricopa past what is now the Carrizo Plain National Monument and crossing the axis of the San Andreas Fault into the southern San Joaquin Valley.

The route was subsequently realigned several times, most notably the original Ojai Freeway in southern Ventura which is now the modern SR 33 freeway, and the expressway bypass of eastern Taft which is now the modern SR 119 expressway.

In the 1920s, the legislative definition would truncate the route as between Ventura and the Cuyama Valley.