Major cities Bolded cities are officially-designated control cities for signs
*Newport Beach (southbound) *Costa Mesa *Santa Ana *Tustin *Orange *Anaheim (northbound) *Riverside (northbound; via SR 91)

State Route 55, known locally as The 55, mostly traversing the Costa Mesa Freeway (formerly the Newport Freeway) is an 18 mile (30 km) long north-south highway in Orange County, California running between Newport Beach in the south and State Route 91 at Anaheim in the north. Starting at Finley Avenue on Newport Boulevard in Newport Beach, 0.3 mile south of State Route 1, Route 55/Newport Boulevard is a 4-lane expressway for approximately three-quarters of a mile to its intersection with 17th Street in Costa Mesa. It then follows a traditional street routing through a retail and commerical section of Costa Mesa until its intersection with 19th Street. Following the 19th Street intersection, Route 55 becomes an eight-lane below-grade freeway that bisects the northbound and southbound lanes of Newport Boulevard until the Mesa Drive undercrossing. North of Fair Drive, Route 55 is an at-grade or above-grade freeway, with the exception of a one mile stretch between the 1st Street/4th Street exit and the 17th Street exit in Santa Ana which is below-grade.

The entire freeway section of Route 55, approximately 16 miles, is called the Costa Mesa Freeway. It was formerly called the Newport Freeway, with the name being changed by the legislature in 1976. Caltrans did not begin changing signs to reflect the name change until the early 1990s.

The segment on Newport Boulevard includes a limited-access interchange at State Route 1. The southbound side of the Costa Mesa Freeway does not have a direct link to northbound Interstate 5, the Santa Ana Freeway. Route 55 is known to be crowded due to it being the primary free link from job centers in Irvine, Santa Ana and Costa Mesa to State Route 91, which leads to commuter communities of the Inland Empire and other places northeast of Orange County.

The 55 was the first freeway in Orange County to receive carpool lanes, opened in 1985. The stretch of the 55 between Fair Dr. and 19th Street in Costa Mesa was opened in 1990; plans to extend the 55 freeway south from 19th Street to State Route 1 were never realized, but in April of 2007, the Orange County Transportation Authority approved funds to study the feasibility of extending the southern freeway portion of Route 55 in Costa Mesa south to 17th Street via tunnels or flyover ramps.

This route is part of the California Freeway and Expressway System CA Codes (shc:250-257) .