U.S. Highway 20 (US 20) is the portion of an east–west United States Numbered Highway in the state of Idaho. It begins northwest of Parma at the Oregon state line and enters Montana 9.6 miles (15.4 km) away from the Yellowstone National Park west entrance.

Since 2019, US 20 has been designated as the Idaho Medal of Honor Highway.

At the western border, adjacent to Nyssa, Oregon, US 20 (running concurrently with US 26) crosses the Snake River into Idaho at an approximate elevation of 2,200 feet (670 m) above sea level. It joins US 95 and runs southbound to Parma. US 20/US 26 leaves US 95 southeast of Parma and runs to Caldwell, where it briefly joins Interstate 84 (I-84; and US 30) from exit 26 until exit 29.

These four highways parallel each other east (on two roadways) to Boise where US 20/US 26 runs through downtown, then southbound on Broadway Avenue to rejoin with I-84/US 30 at exit 54. The four concurrent routes head southeast to Mountain Home, where US 20 departs at exit 95 to head east, past Rattlesnake Station, Anderson Ranch Dam road, and crests at Cat Creek summit at 5,527 feet (1,685 m). This portion of the route is near Goodale's Cutoff of the Oregon Trail.

US 20 continues east, into, and across Camas County through Fairfield to Timmerman Junction, the intersection in Blaine County with State Highway 75 (SH-75), the northbound route to Sun Valley, Galena Summit, and Stanley. US 20/US 26 goes through Picabo and Carey, joins with US 93, and on to Craters of the Moon National Monument and Preserve and Arco, where US 93 splits off and turns north-northwest to climb the Big Lost River valley.

US 20/US 26 continues on through Idaho National Laboratory, where the highways split just west of Atomic City; US 26 heads southeast to Blackfoot and US 20 to Idaho Falls, where it turns north-northeast to pass near Rexburg as a freeway. US 20 then climbs through the communities of St. Anthony, Ashton, and Island Park, and crosses the Continental Divide at Targhee Pass at 7,072 feet (2,156 m), entering Montana west of West Yellowstone.

This section needs expansion. You can help by adding to it. (March 2017)

US 20 was expanded past Yellowstone National Park in 1940, replacing the original US 191 and several state highways. Its original alignment through Boise included Main Street and Fairview Avenue for westbound and eastbound traffic, respectively, and Capitol Boulevard. US 20 and US 26 were realigned onto Myrtle and Front streets as well as Broadway following the completion of the Boise Connector extension on August 7, 1992.

The Rigby Freeway section of US 20, extending from Idaho Falls to St. Anthony, was constructed in the 1970s. Its northernmost section still featured at-grade intersections until the 2000s, when they were replaced with full interchanges, though a few at grade crossings still exist between Sugar City and St. Anthony as of September 2023.

This section is missing mileposts for junctions. Please help by adding them.

County	Location	mi	km	Exit	Destinations	Notes
Snake River		0.000	0.000		US 20 west / US 26 west – Nyssa	Continuation into Oregon
					Snake River Bridge; Oregon–Idaho state line
Payette	​	1.578	2.540		US 95 north – Fruitland, Payette	Western end of US 95 concurrency
Canyon	​	9.647	15.525		US 95 south – Jordan Valley	Eastern end of US 95 concurrency
	Caldwell	21.995– 22.129	35.398– 35.613		I-84 / US 30 west – Ontario	Western end of I-84/US 30 concurrency; I-84 exit 26
				27	I-84 BL east to SH-19 – Caldwell, Wilder, Homedale	Exit numbers follow I-84
				28	10th Avenue – City Center
		24.840– 24.994	39.976– 40.224		I-84 / US 30 east – Nampa, Boise	Eastern end of I-84/US 30 concurrency; I-84 exit 29
Ada	​	34.267	55.147		SH-16 – Emmett
	Boise–Eagle line	40.229	64.742		SH-55 – Caldwell, Eagle
	Garden City	44.166	71.078		SH-44 – Eagle
	Boise				Main Street	Western end of freeway; westbound entrance only
		47.300	76.122	—	I-184 west to I-84	Eastern terminus of I-184; westbound left exit and eastbound left entrance
		48.050	77.329	—	River Street	Eastbound exit only
				—	Front Street west	Eastern end of freeway; westbound exit only
		51.975	83.646		Federal Way	Interchange
		52.560– 52.664	84.587– 84.754		I-84 / US 30 west / Commerce Avenue – Nampa	Western end of I-84/US 30 concurrency; I-84 exit 54
				57	SH-21 (Gowen Road) – Idaho City	Exit numbers follow I-84
	​			59	S. Eisenman Road, Memory Road	Signed as exits 59A (S. Eisenman Road) and 59B (Memory Road) eastbound
	​			64	Blacks Creek Road – Kuna
	Regina			71	Mayfield, Orchard
Elmore	​			74	Simco Road
	​			90	I-84 BL east to SH-51 / SH-67 – Mountain Home, Bruneau	Eastbound signage
					West Mountain Home	Westbound signage
	Mountain Home	95.066– 95.307	152.994– 153.382		I-84 / US 26 / US 30 east / I-84 BL west (SH-51) – Mountain Home, Twin Falls, Air Force Base, Duck Valley	Eastern end of I-84/US 26/US 30 concurrency; I-84 exit 95
Camas	Fairfield				SH-46 – Gooding, Wendell
Blaine	​				SH-75 – Shoshone, Challis
	Carey				US 26 / US 93 – Bliss, Twin Falls	Western end of US 26/US 93 concurrency
Butte	Arco				US 93 – Challis	Eastern end of US 93 concurrency
	Butte City				SH-33 – Howe
	Atomic City				US 26 – Idaho Falls	Eastern end of US 26 concurrency
Bingham	No major junctions
Bonneville	Idaho Falls			I-15 south / I-15 BL / US 20 Bus. (Broadway Street east) to US 26 – Blackfoot, Pocatello, Jackson, Idaho Falls	Western end of I-15 concurrency; I-15 exit 118
				I-15 north – Roberts, Butte, Airport	Eastern end of I-15 concurrency; I-15 exit 119; access to airport via Grandview Avenue; signalized interchange
				Western end of Rigby Freeway
				307	Lindsay Boulevard
				308A	Riverside Drive – City Center	Signed as Exit 308 eastbound
				308B	Science Center Drive	Eastbound exit and westbound entrance
	​			310	US 20 Bus. – Lewisville
	Orvin			311	Saint Leon Road, 15th East
	​			313	Hitt Road, 25th East
	Ucon			315	SH-43 – Ucon
Bonneville–Jefferson county line	​			318	County Line Road
Jefferson	Rigby			320	US 20 Bus. to SH-48 – Rigby	Eastbound exit and westbound entrance
				322	US 20 Bus. – North Rigby	Eastbound signage
					US 20 Bus. to SH-48 – Rigby	Westbound signage
	Lorenzo			325	Menan, Roberts
Snake River				Bridge
Madison	​			328	Thornton
	Rexburg			332	BYU-Idaho, South Rexburg
				333	SH-33 – Salmon, Rexburg
				337	North Rexburg
	Sugar City			338	Sugar City, Salem	Eastbound exit and westbound entrance
				339	To SH-33 east – Driggs, Jackson, Sugar City	Eastern end of Rigby Freeway
Fremont	​			344	South St. Anthony	New interchange under construction
	St. Anthony			346	US 20 Bus. east – St. Anthony	Interchange
	​				US 20 Bus. west – St. Anthony
	Ashton				SH-47 to SH-32 – Driggs
	Island Park				SH-87 – Ennis
Continental Divide					Targhee Pass; Idaho–Montana state line
					US 20 east – West Yellowstone, Bozeman	Continuation into Montana
1.000 mi = 1.609 km; 1.000 km = 0.621 mi       Concurrency terminus       Incomplete access       Route transition

U.S. Route 20
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