It bypasses Seattle east of Lake Washington, traveling through the Eastside area of King and Snohomish counties, providing an alternate route to I-5.

A freeway replacement for SSH 2A was proposed in the 1940s by the state government and designated as I-405 as part of the federal Interstate Highway program, with the first section beginning construction in 1956 and completed in 1965.

It was initially signed as SR 405 until the freeway was fully completed in 1971; since then, the highway has been expanded to add lanes for high-occupancy vehicles and toll users.

[9] Tolls are collected electronically through Good to Go transponders by overhead sensors or via license plate cameras for mail billing with a surcharge.

[14] The eight-lane freeway gains a set of HOV lanes, directly connected to I-5, and travels around the north side of the Southcenter Mall, at the center of Tukwila's retail district.

[15] I-405 travels east across the Green River and intersects Interurban Avenue at the north end of SR 181, located near the Tukwila train station and Starfire Sports soccer complex at Fort Dent Park.

[18][19] I-405 continues across the Black River and through an industrial and commercial area on the southern outskirts of Renton to a cloverleaf interchange with SR 167 (the Valley Freeway) and Rainier Avenue.

[24] The freeway, now closely following the shore of Lake Washington and the Eastside Rail Corridor trail,[27] passes the Virginia Mason Athletic Center, the headquarters of the Seattle Seahawks football team.

I-405 then passes the Marketplace at Factoria shopping center and the headquarters of T-Mobile US before it reaches a full stack interchange with I-90, which provides access to Seattle, Mercer Island, Eastgate, and Issaquah.

[14][20] The freeway continues north from the interchange towards Downtown Bellevue, passing the Mercer Slough estuary and the historic Wilburton Trestle as it widens to ten lanes.

[31] I-405 travels through several interchanges and forms the eastern border of Downtown Bellevue, a major office district, separating it from the retail areas of Wilburton.

[36][37] I-405 continues north from Downtown Bellevue into the city's northern residential and commercial neighborhoods, where it intersects SR 520—a major east–west freeway with connections to Seattle, the Microsoft campus in Overlake, and Redmond.

The freeway leaves Bellevue for Kirkland, traveling along the west side of Bridle Trails State Park and passing the campus of Northwest University in the predominantly residential Houghton neighborhood.

[38] I-405 then travels along the west side of Rose Hill, an area uphill from and about one mile (1.6 km) east of downtown Kirkland, intersecting Northeast 85th Street (formerly SR 908) near Lake Washington High School.

[46] The freeway continues northwest through residential neighborhoods and gains a set of auxiliary lanes in the shoulder that are opened to northbound traffic during limited periods using a series of overhead signs and restricted to transit use in the southbound direction.

[14] During early development of the Eastside region in the late 19th century, travel between cities was dependent on boats, including a system of passenger steamboats and ferries on Lake Washington.

[51][52] The first completed railroad on the Eastside was the Lake Washington Belt Line, opened from Renton to Woodinville in late 1904 by the Northern Pacific Railway.

[79][80] In 1957, the Washington State Highway Commission approved accelerated plans to build new two-lane sections of the future freeway corridor that would be designed to accommodate later expansions to four lanes with limited access to adhere to Interstate standards.

[87][88] The highway's completion spurred economic growth in Bellevue, which was sought due to its location near a junction with US 10; from 1955 to 1958, the city and its surrounding areas had grown from a population of 27,600 to 41,750.

[90] A nearby interchange at Northrup Road was rebuilt in 1963 to temporarily accommodate traffic from the then-new Evergreen Point Floating Bridge using a hybrid cloverleaf design.

[100][101] The 209-acre (85 ha) interchange with I-5 at Tukwila opened in stages between February and November 1967;[102][103] it cost $16 million to construct and included ramps to the new Southcenter shopping mall.

Also during this time, the portion of the highway between I-5 and South Renton was being repaved and HOV lanes were being added,[115] which were originally placed on the right-hand side of the road before being migrated over to the median in the early 2000s.

[119] A 2008 construction project demolished the Wilburton Tunnel, which formerly covered a portion of the freeway between Factoria and Bellevue and carried the BNSF Woodinville Subdivision.

[121][122] Construction on the southern half of the HOT lanes system, extending to the SR 167 interchange in Renton, began in 2020 and is scheduled to be completed in 2024 at a cost of $1.2 billion.

[133][134] One of the new stations on the BRT corridor, at Northeast 85th Street in Kirkland, is planned to include reconstruction of the existing cloverleaf interchange at an estimated cost of $300 million.