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.

[7][8] 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.

[8] 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.

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.

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.

[7][8] 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.

[8] 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.

[7] The freeway travels around the south side of Wendell and turns east to follow an active railroad through a rural area that marks the boundary of Jerome County.

The interstate intersects SH-79 on the south side of Jerome and reaches a partial cloverleaf interchange with US 93 across the Snake River Canyon from the city of Twin Falls.

At a partial cloverleaf interchange in northeastern Heyburn, the freeway intersects SH-24 and is rejoined by US 30, beginning a new concurrency as the highway leaves the Magic Valley.

Near the Minidoka National Wildlife Refuge, I-84 reaches the western terminus of I-86, which carries US 30 and follows the Snake River upstream to American Falls and Pocatello.

[7][8] After crossing the rural valley, I-84 begins following Meadow Creek and climbs into Mortenson Canyon, which marks the northeastern extent of the Black Pine Mountains.

The highway crests at Sweetzer Summit, located 5,522 feet (1,683 m) above sea level, and descends into the Juniper Valley on the south side of the Sublett Range in Sawtooth National Forest.

[13][14] I-84 continues southeast across the valley, passing a set of rest areas in Oneida County, and reaches the Utah state border northwest of Snowville.

[24] Another section was opened around the same time in Sublett in eastern Cassia County, which caused the killing of eighteen mule deer from a local herd from collisions with drivers within the first six weeks.

[29] To address increasing traffic congestion in the Boise metropolitan area, fueled by population growth and suburban development, the Idaho Transportation Department (ITD) has widened 24 miles (39 km) of I-84 with funds from a bond sale approved by the state legislature in 2005.