Interstate 780 (I-780) is an Interstate Highway in the San Francisco Bay Area of the U.S. state of California. It serves to connect Interstate 80 in Vallejo with Interstate 680 just north of the Benicia-Martinez Bridge in Benicia. Originally, this segment was part of I-680 before that Interstate was extended and rerouted to Cordelia. The city-maintained Curtola Parkway continues west from I-80 to State Route 29 in Vallejo.

The Interstate 780 state-maintained freeway begins at Lemon Street and Curtola Parkway, quickly crossing underneath I-80 at a cloverleaf interchange. The freeway passes through parts of unincorporated Solano County, then heads southeast along the Benicia State Recreation Area (a marsh). It then bypasses downtown Benicia through the hills to the north. I-780 ends at I-680 at the north end of the Benicia-Martinez Bridge, with I-680 heading south through the East Bay to San Jose.

By 1914, a paved county road connected Vallejo and Benicia north of the Carquinez Strait, following the present Maine Street, Benicia Road, Columbus Parkway, and K Street. Although state highways were designated to Benicia in 1910 (Route 7, now I-680) and Vallejo in 1931 (Route 74, now SR 29), this connection was not added until 1935, when Route 74 (an unsigned designation) was extended east from Vallejo to Benicia and north along former Route 7 to Cordelia. (A short spur connecting Vallejo to the Carquinez Bridge was added to Route 74 in 1937.) Route 74 initially entered Benicia on K Street and left on East Fifth Street, making several turns in between. Two realignments were built in the 1940s, reducing the number of turns to one, at the corner of L and East Second Streets.

On September 15, 1955, the Bureau of Public Roads approved the Interstate Highway System spurs and connections in urban areas. Among these was a loop around the San Francisco Bay, soon numbered I-280 and I-680. The east half (I-680) incorporated a number of existing legislative routes, including Routes 69, 108, 107, 75, and 74, crossing the Carquinez Strait on the proposed Benicia-Martinez Bridge and ending at I-80 near downtown Vallejo. The first piece of this freeway north of the Carquinez Strait was at the I-80 cloverleaf interchange, built in the late 1950s when I-80 was upgraded through Vallejo. This was an extremely short roadway, beginning at Lemon Street, crossing under I-80 and Laurel Street, and ending at Reis Avenue and Cedar Street. In about 1960, it was extended east to the old highway (Columbus Parkway) between the cities, and was completed to the new bridge in about 1962, the year the bridge opened.

In the 1964 renumbering, the legislative designation of the completed Vallejo-Benicia freeway was changed from Route 74 to Route 680, reflecting its Interstate designation. The short piece of non-freeway in Vallejo, extending west from I-80 to SR 29, became State Route 141. This route followed Benicia Road and Maine Street, the same alignment the state highway had always taken; it initially connected with I-780 via Lemon Street, but later followed Benicia Road from I-80 north of I-780. In 1975, a proposed (and never constructed) extension west and north to SR 37 east of the Napa River was added to Route 141. The route was to be the Waterfront Freeway, scaled down to a boulevard in 1974.

The Federal Highway Administration approved a relocation of I-680 onto the State Route 21 freeway between Benicia and Cordelia in July 1973. To keep the route to Vallejo in the Interstate system, it was renumbered I-780; the corresponding legislative changes were made in 1976. As part of the project to construct a new northbound Benicia-Martinez Bridge, the I-680/I-780 interchange was rebuilt;[when?] the new span opened in August 2007. Route 141 was deleted from the state highway system in 1988, soon after the city of Vallejo constructed Curtola Parkway (partially replacing Maryland Street) just to the south, directly connecting I-780 with a surface road to SR 29.