State Route 15 (SR 15) in the U.S. state of California is the seamless 5.59-mile (9.00 km) freeway continuation of Interstate 15 past its southern terminus at Interstate 8 in San Diego to the Interstate 5 junction 12 miles (19 km) from the Mexican border. The Route 15 designation is also used by Caltrans for the full length of I-15.

SR 15 is part of the California Freeway and Expressway System. It continues seamlessly past the southern terminus of I-15 at I-8 in San Diego. On the northbound conversion to I-15 at I-8, there is no "End SR 15" sign. Between I-8 and I-805, SR 15 follows the former alignment of 40th Street, which was its former routing as a city street. Between the Polk Avenue and Orange Avenue overpasses, the freeway goes under a city park that was built on top of the freeway during construction in 2001. Pedestrian bridges were also built at Monroe Avenue and Landis Street to reduce the effects of the freeway geographically dividing the community. The route interchanges with Interstate 805; however, one can only continue in the same direction that they were at this intersection, since this is the interchange of two north–south freeways, resulting in the overall shape of an "X." After this, SR 15 has an interchange with State Route 94, which has been cited as not being up to Interstate standards. Then, SR 15 terminates south of Interstate 5 at 32nd Street near Harbor Drive.

Present SR 15 was signed after the creation of I-15 in 1968. Since I-15's terminus was at I-8, SR 15 was signed mostly along 40th Street and Wabash Boulevard in San Diego to its merge with I-5. The portion between Adams Avenue and Interstate 805 remained a city street for a long time, and this portion was not completed until January 2000. For this reason, the freeway is often referred to as the 40th Street Freeway.

Before the completion of the freeway, from 1968 to 1992, the neighborhood was known for prostitution, drugs, drive-by shootings, and gangs. The indirect cause of all this was due to Caltrans' plans to build a freeway in this area on land where houses were. Because families did not want to live in these houses since they would be soon torn down, they rented them to those who were only going to be in the area on a temporary basis; many of these people were involved in illegal activities. Even though the freeway was officially added to the Caltrans proposals as early as 1968, it was not until March 1992 that construction began. Many in the city opposed the building of this freeway, although some petitioned for the freeway to be built because of the poor conditions in the neighborhood.

Redesignation of SR 15 as I-15 will eventually occur when the freeway's interchange with SR 94 is upgraded to Interstate standards. The interchange currently has left-exits and blind merges, and is due to be updated with a long-awaited widening of both SR 15 and SR 94 by 2016. At that time SR 15 will be resigned as part of I-15. The remaining portion of SR 15 conforms with Interstate standards.

The entire route is in San Diego, San Diego County.