Interstate 84 (I-84) in the U.S. state of Idaho is a major Interstate Highway that traverses the state from the Oregon state line in the northwest to Utah state line in the southeast. It primarily follows the Snake River across a plain that includes the cities of Caldwell, Nampa, Boise, Mountain Home, and Twin Falls. The highway is one of the busiest in Idaho and is designated as the Vietnam Veterans Memorial Highway.

I-84 runs for 276 miles (444 km) within Idaho, beginning near Ontario, Oregon, and traveling concurrent with several U.S. routes through the Boise metropolitan area and Mountain Home towards Twin Falls. I-84 splits away from US 30 and the Snake River at a junction with I-86 near Declo, where it turns southeast to cross the Sublett Range into northern Utah. The highway has an auxiliary route, I-184, which serves downtown Boise.

I-84 is the longest Interstate highway in Idaho, running for 276 miles (444 km) and connecting several of the state's largest metropolitan areas. It has a single auxiliary route, I-184 in Boise, and several business routes. The highway was officially designated as the Vietnam Veterans Memorial Highway in 2014, mirroring the name for Oregon's section of I-84. The Idaho section of I-84 is maintained by the Idaho Transportation Department (ITD), which conducts an annual survey of traffic on certain highway segments that is expressed in terms of annual average daily traffic (AADT), a measure of traffic volume for any average day of the year. Average daily traffic volumes on I-84 in 2018 ranged from a minimum of 9,552 vehicles near Sweetzer Summit and 139,113 vehicles at I-184 in western Boise.

I-84 enters the state of Idaho after crossing the Snake River from Ontario, Oregon, west of Fruitland, Idaho. The four-lane freeway travels southeast along the Snake River and passes an eastbound-only rest area and visitors center, which mirrors a westbound facility on the Oregon side of the river. Its first interchange is with US 95, the primary north–south highway in Idaho, at Palisades Corner south of Fruitland. I-84 continues southeast across farmland in the Snake River Plain and straddles the edge of a hill. South of New Plymouth the freeway intersects and becomes concurrent with US 30. The paired highways continue south along the side of a hill and cross into Canyon County northeast of Notus.

The freeway enters the Treasure Valley, which also constitutes much of the Boise metropolitan area, near the outskirts of Caldwell and intersects State Highway 44 (SH-44), which continues east to Middleton and Eagle. Shortly before crossing the Boise River into Downtown Caldwell, I-84 and US 30 begin a brief concurrency with US 20 and US 26, with both continue west towards Parma and the Oregon state border. The freeway skirts the north side of Downtown Caldwell, which is served by a designated business route, and passes through residential neighborhoods before US 20 and US 26 split off and travel east onto Franklin Road. I-84 and US 30 continue southeast through an industrial area and pass the Caldwell Industrial Airport and Karcher Mall before reaching the city of Nampa. The six-lane freeway travels through an industrial area on the northwestern side of Nampa, parallel to a set of railroad tracks, but turns east to bypass the city center. SH-55 joins the concurrency at an interchange with Karcher Road and I-84 Business returns to the freeway at an interchange located near Nampa Gateway Center, the Nampa campus of the College of Western Idaho, and the Ford Idaho Center.

I-84 expands to eight lanes as it leaves Nampa and enters Ada County, passing through suburban neighborhoods in Meridian. The freeway crosses under two single-point urban interchanges at Ten Mile Road and SH-69, along with a partial cloverleaf interchange where SH-55 splits from the concurrency to head north. I-84 enters the city of Boise and expands to ten lanes before an interchange with I-184, an auxiliary route that travels into Downtown Boise. I-84 turns southeast and travels around the city's outer residential neighborhoods, passing the main terminal at Boise Airport and a nearby industrial area. The freeway is rejoined by US 20 and US 26 before leaving Boise, climbing out of Treasure Valley by following Fivemile Creek over a barren plateau.

The highway crosses into Elmore County and rejoins the Union Pacific Railroad's Nampa Subdivision as it passes the Morley Nelson Snake River Birds of Prey National Conservation Area. I-84 then travels around the city of Mountain Home, while a signed business route serves the city center and leads to the nearby Mountain Home Air Force Base. On the east side of the city, I-84 intersects SH-51 and ends its concurrency with US 20, which continues east towards Sun Valley, Idaho Falls, and Yellowstone National Park. I-84, US 26, and US 30 continue southeast through the conservation area and descends from the plateau into the Snake River Plain. The freeway travels around the north side of Hammett, which is served by a business route that connects to SH-78, and reaches the Snake River.

I-84 travels upstream along the north bank of the Snake River to Glenns Ferry, where it bypasses the town and Three Island Crossing State Park. The freeway crosses over the river twice east of Glenns Ferry to avoid a bend in the river, which is followed by the railroad. After entering Gooding County, I-84 reaches the town of Bliss, where US 30 and US 26 both split from the freeway and end their concurrency. US 30 travels south along the Snake River towards Hagerman, while US 26 runs east across the plains to Gooding and Craters of the Moon National Monument and Preserve. The freeway continues southeast and crosses 250 feet (76 m) over the Mallad Gorge (part of Thousand Springs State Park) on a pair of level bridges. I-84 then passes through farmland on the edge of a lava plain while following an abandoned railroad right of way to Wendell, where it intersects SH-46.