State Route 33 (SR 33) is a north–south state highway in the U.S. state of California. SR 33 replaced part of U.S. Route 399 in 1964 during the "great renumbering" of routes. In the unincorporated sections of Kern County it is known as the West Side Highway. In addition, the California Legislature designated the entire Kern County portion as the Petroleum Highway in 2004. The southernmost portion is a freeway known as the Ojai Freeway.

Several segments of this route are part of the California Freeway and Expressway System; also, several are eligible to be included in the State Scenic Highway System. However, only the portion from 6.4 miles north of the SR 150 junction to the Ventura - Santa Barbara county line is a scenic highway as designated by Caltrans.

Beginning in Ventura as a short freeway (the "Ojai Freeway"), Route 33 continues north as an undivided surface road through Ojai, follows Matilija Canyon past the easternmost extremity of the Santa Ynez Mountains, and passes over the Transverse Ranges at Pine Mountain Summit, coming down into the Cuyama Valley to an eventual junction with SR 166. Route 33 continues with Route 166 for about 20 miles (32 km) to Maricopa where the routes separate. In Maricopa, Route 33 turns northwest, following the western side of the San Joaquin Valley. Between Taft and Avenal it roughly parallels Interstate 5; for the first 15 miles (24 km) of this portion it follows the axis of the Midway-Sunset Oil Field. Northwest of Avenal, it turns due west to enter Coalinga, then turns northeast and soon becomes concurrent with I-5 for 13 miles (21 km). Thereafter, it departs due north near Cantua Creek, turns northwest at Mendota, and meets State Route 152 in Dos Palos, with which it runs concurrently for approximately ten miles. It departs again to the northwest near Los Banos, turns north at Santa Nella (where it has an interchange with I-5), and roughly parallels I-5 and the San Joaquin River until its final terminus at I-5 at Yarmouth.

When passing through the Avenal/Coalinga area travelers pass near a series of prisons and government facilities. They include: Avenal State Prison, Pleasant Valley State Prison, and the new Coalinga Mental Hospital.

In the portions of the route in Kern and Fresno Counties, SR 33 passes through one of the United States' largest petroleum extraction fields, with hundreds of nodding oil pumps along the highway. Some of the fields visible from the highway include the Midway-Sunset, South and North Belridge, Cymric, McKittrick, Lost Hills, and Coalinga oil fields.

Most of SR 33's route is through sparsely populated, relatively desolate portions of the San Joaquin Valley. Travelers along it should use caution, since emergency services are typically dozens of miles away. Carrying extra water and coolant is especially advised, since summertime temperatures in the area routinely surpass 100 °F (37 °C).

Before the US 399 designation was deleted in 1964, SR 33 followed SR 166 east from US 99 at Wheeler Ridge to Maricopa in lieu of the segment from Ventura to Maricopa, and followed the old US highway from Taft to Maricopa. Today, SR 166 remains on the segment between Wheeler Ridge and Maricopa.

Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, and do not necessarily reflect current mileage. The numbers reset at county lines; the start and end postmiles in each county are given in the county column.