State Route 84 (SR 84) is a split-section California State Highway consisting of two sections. The first section is an east–west arterial road running from San Gregorio to Menlo Park, across the Dumbarton Bridge through Fremont and Newark and ending at I-580 in Livermore. The route overlaps the freeway segment of U.S. 101 between Woodside Road in Redwood City and Marsh Road in Menlo Park. The segment between Marsh Road and the Dumbarton Bridge has been upgraded to an expressway and is known as the Bayfront Expressway. The segment from the western end of the Dumbarton Bridge to the interchange with I-880 has been upgraded to a freeway.

The other section is a north-south arterial road that begins at Route 12 in Rio Vista, passes through Ryer Island (where it connects to Route 220), and ends at the Interstate 80 interchange in West Sacramento. However, plans are underway to turn maintenance of SR 84 through West Sacramento to that city.

A ferry provides the crossing over Cache Slough from Rio Vista to Ryer Island. The ferry, a diesel-powered boat operated by Caltrans, is in operation twenty-four hours per day and charges no toll.

Route 84 is legally defined to continue from I-580 to Route 4 in Antioch, but there are currently no plans in place to bridge the gap at this time. A plan to build a $600 million toll road called the Mid-State Tollway along the proposed route was suspended in 2001 due to local opposition.

This route is part of the California Freeway and Expressway System and is eligible for the State Scenic Highway System. However, it is not a scenic highway as defined by Caltrans.

The route begins at SR 1 in San Gregorio. It then heads northward through San Mateo County as it enters Woodside, where it intersects SR 35, also known as Skyline Boulevard, and interchanges with Interstate 280 (California). It then enters Redwood City, where it intersects SR 82, which carries El Camino Real through the south bay area. A few miles later, it interchanges with U.S. Route 101, which it overlaps for several miles. Upon exiting, it enters the city of Menlo Park as an expressway, where it intersects SR 114 and SR 109 at intersections controlled by traffic lights. The SR 114 (Willow Road) intersection was the site of the car crash in which author David Halberstam was killed on April 23, 2007.

Route 84 then becomes a freeway at the south end of San Mateo County as it crosses as the Dumbarton Bridge over the San Francisco Bay. Midway over the bridge, it enters Alameda County.

In Alameda County, it runs northward through the city of Newark, where it begins a concurrency with Interstate 880. Both interchanges with Interstate 880 are partial cloverleaf interchanges, meaning that through traffic on Route 84 encounters traffic lights while joining and leaving I-880. Upon separation, the route loses its freeway status as it enters the city of Fremont, where it has a short concurrency with SR 238. There are plans to construct a new parkway between the I-880 interchange and Mission Boulevard (CA 238).

It then leaves Fremont through the historic Sunol Valley. After exiting the valley, it begins an overlap with I-680. After separating, it runs for several miles over a pass, then enters the city of Livermore; it runs along Isabel Ave until it meets I-580, as the end of its southern section.

Upon reappearing, the route begins at SR 12 in Rio Vista. In then follows the Ryer Island Ferry, which carries the route across the Cache Slough. When it leaves the ferry, it intersects SR 220. It continues northward and meets the end of state maintenance at the West Sacramento city limit, about six miles east of an intersection with CR E19. It then enters the city of West Sacramento in Yolo County. It then interchanges with I-80 Business, also signed as U.S. Route 50. As it continues northward, it ends at Interstate 80.

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.