U.S. Highway 20 (US 20) is the portion of an east–west United States Numbered Highway in the state of Illinois. After crossing the Mississippi River, US 20 enters the state in East Dubuque and runs in a generally east-southeast direction, exiting in the East Side neighborhood of Chicago. Since 2022, the highway has been officially named the Illinois Medal of Honor Highway, though it is also signed as the General Ulysses S. Grant Memorial Highway (often abbreviated as the Grant Memorial Highway).

As it travels through Illinois in the northern region of the state, US 20 passes through 23 state highways. Its routing joins three US highways (US 12, US 41, and US 45) and three interstate highways (I-39, I-290, and I-294). US 20 serves as a terminus point for both the shortest Illinois highway (Illinois 35) and the longest (Illinois 1).

Within Illinois, US 20 runs for a distance of , its third-longest span east of the Mississippi River.

Route description As it travels through Illinois, US 20 varies widely in its configuration. Prior to entering the Chicago suburbs, the majority of the route is a two-lane surface road connecting small rural towns and multiple north-south state highways. In Freeport, the route expands to a controlled-access freeway bypass supplementing a business route. In Rockford, US 20 not only has a freeway bypass and a business route (the latter, expanding between four and six lanes), but the route plays a significant role in linking the city together with other locations in the region. US 20 is routed through Elgin with a four-lane bypass, connecting to several major roads.

After exiting Elgin as Lake Street, US 20 continues through the rest of the state as an arterial surface road with either four or six lanes.

Jo Daviess and Stephenson Counties

Crossing the Mississippi River on the Julien Dubuque Bridge, US 20 enters Jo Daviess County in East Dubuque, crossing Illinois 35 (the shortest state highway in Illinois) directly as it exits the bridge. Surrounded by tall hills to the north and the river to the south, the highway expands from two lanes to a four-lane divided highway. Upon exiting the city, it climbs uphill into the rugged country of the Driftless Area. Approximately 7 miles after exiting East Dubuque, Illinois 84 overlaps US 20 as the route turns southward towards Galena. Entering the city, the route (as Spring Street) steeply descends towards the Galena River; after crossing the river, the route (renamed as US 20 within Galena) climbs steeply upward, passing one block south of the Ulysses S. Grant Home. After exiting Galena, US 20 winds in a general southeast direction away from the Wisconsin border, passing through a mix of heavily wooded surroundings and rugged land with steep bluffs. After US 20 reaches its highest point in Illinois, the route ends its concurrency with Illinois 84 approximately 2 miles west of Elizabeth. While not specifically reserved for truck traffic, uphill portions of US 20 in Jo Daviess County are fitted with passing lanes (over a mile long), allowing for vehicles to pass each other safely without moving into oncoming traffic. US 20 exits the Driftless Area as it turns towards Stockton, turning directly eastward, where it carries Illinois 78 for slightly over a mile (before the latter moves northward).

Entering Stephenson County, US 20 moves into (relatively) smoother terrain, surrounded entirely by farmland. As it crosses Illinois 73, the route passes directly south of Lena; on its way to Freeport, the route loosely parallels Canadian National tracks (routing downtown Chicago to Iowa through Rockford). At a signal-controlled intersection outside of Freeport, US 20 splits into a business route entering the city as its primary east-west street (Galena Avenue) and a four-lane controlled-access freeway, bypassing Freeport to the north and to the east. Along with crossing the Pecatonica River, both routings connect Freeport with Illinois 26 and Illinois 75. After exiting the city, the business route rejoins US 20, reconfigured as a four-lane divided highway. Again surrounded completely by farmland, the highway passes between Ridott and German Valley before exiting the county.

Winnebago and Boone Counties

US 20 enters Winnebago County 10 miles westward of Rockford (the largest city in Illinois outside of Chicago and its suburbs). Before entering the city, the route passes through signal-controlled intersections: Pecatonica Road (south of Pecatonica) and at Winnebago Road/Elida Street in Winnebago. Two miles east of the latter intersection, US 20 enters Rockford, splitting into a four-lane controlled-access freeway bypass and a four-lane surface street (State Street; US 20 Business); the latter serves both Rockford and Belvidere.

Between Rockford and Cherry Valley, US 20 splits from I-39/US 51 before the two interstates link together. This is the point where US 20 and I-90 begin to parallel each other, doing so until their eastern terminus in Massachusetts.

Rockford On the US 20 bypass, the four-lane freeway turns southeast of Rockford approximately 2 miles east of Winnebago; it has 6 interchanges with major city streets. Before its interchange with Illinois 2 (Main Street), the bypass turns directly east, subsequently passing Illinois 251, before taking on I-39/US 51. After passing South Alpine Road, US 20 passes an interchange with Interstate 39, with the eastern portion of the US 20 bypass also concurrent with US 51. US 20 exits the bypass between Rockford and Cherry Valley in its interchange with Harrison Avenue (I-39 exit 122); less than a mile northward, the bypass has its northern terminus, as I-39/US 51 merge with I-90 (I-90 Exit 17).

The US 20 business route enters Rockford at its western edge as West State Street, traveling towards the downtown region of the city through mixed residential and commercial developments. After crossing the southern terminus of Illinois 70 (Kilburn Avenue), the business route designation is moved from State Street (which narrows to two lanes) to two separate three-lane roads: Jefferson Street (westbound traffic) and Chestnut/Walnut Street and 1st Avenue (eastbound traffic). After crossing the Rock River, the business route returns to East State Street a block east of Charles Street, shortly after it exits downtown and returns to four-lane traffic. Initially surrounded by residential neighborhoods, the route shifts towards entirely towards commercial developments, widening to six lanes. After passing under I-39/90, it narrows from six lanes to four and exits the city.

Belvidere US 20 continues towards Belvidere in a right turn from the Rockford bypass (taking over for Harrison Avenue). Bordering to the north of Cherry Valley, US 20 passes over the Kishwaukee River before passing underneath I-90 as it heads directly east, paralleling I-90 through Belvidere (forming much of its southern border). The majority of the route is a four-lane divided highway, with US 20 having an interchange with Appleton Road (accessing the Belvidere Assembly Plant). At its intersection with Belvidere Road/Genoa Road, US 20 is the eastern terminus of the Rockford/Belvidere US 20 business route, which is ½-mile north of an I-90 interchange.

After exiting Rockford, the US 20 business route continues as East State Street until the Boone county line, where it becomes signed as "US 20 Business"; in addition to farmland, the route is surrounded by forest preserves and rural subdivisions. Following a signaled intersection, the route narrows down to two lanes. After entering Belvidere, the business route (renamed as North State Street) turns southeast. Within the Belvidere city limits, the route turns southeast and is renamed North State Street. At its intersection with Appleton Road, the business route splits from State Street to form a bypass around Belvidere. Along with forming the southern terminus of Illinois 76, the bypass forms much of the northern and eastern borders of the city; after crossing the Kishwaukee River, it becomes a city street (Belvidere Road), before ending the Rockford/Belvidere business route at its intersection with its US 20 intersection, turning into Genoa Road, which accesses I-90.

Less than one mile from its intersection, US 20 narrows to a two-lane road, crossing its last signaled intersection in the Rockford area a mile later. After passing through the Rockford and Belvidere region, US 20 becomes nearly flat, again surrounded by farmland. Near the county border, the route passes through unincorporated Garden Prairie, inaccessible by I-90.

McHenry and Kane Counties

As it enters McHenry County, US 20 heads straight east as I-90 continues to move southeast. As it enters Marengo, the route retains US 20/Grant Highway as a street name (though serving as its primary east-west road). Near the center of the city, US 20 crosses Illinois 23, linking the route to I-90 (approximately 5 miles south) and the Illinois 176 (its western terminus, ½ mile north); after Illinois 23, the route begins to turn southeast. After passing through unincorporated Coral, US 20 has two large roundabout intersections as it travels towards I-90: south of Union (accessing Marengo, Union, and Huntley) and the center of unincorporated Harmony (accessing IL 23, I-90, and Huntley). Less than a half-mile south of Harmony, US 20 exits the county.

Less than a half-mile south of its roundabout in Harmony, US 20 enters Kane County north of Hampshire. Shortly before crossing I-90 (for a second time), it has a trumpet interchange with the interstate, surrounded by multiple travel centers. After traveling alongside the eastern border of Hampshire, the route turns towards Illinois 47 in Pingree Grove, where US 20 changes routing with Illinois 72 (Higgins Road). As Canadian Pacific rail tracks preclude US 20 forming an intersection with IL 72, Illinois 47 carries the two east-west routes under a railroad viaduct (forming a wrong-way concurrency) for slightly under ½ mile before separating.

Elgin East of Illinois 47, US 20 maintains its southeastern alignment as it travels around Pingree Grove, surrounded by a mix of farmland, residential, and industrial developments. Continuing as a two-lane rural roadway, the route enters the Elgin region as it passes by several residential subdivisions. After crossing Longcommon Parkway, US 20 is expanded to a limited-access freeway (returning to four lanes for the first time since Belvidere), though passing through multiple residential areas on both sides, it largely passes between Elgin and its South Elgin suburb. Along with its first interchange with Randall Road (the historical western border of the Chicago suburbs), the Elgin Bypass accesses Illinois 31 (State Street) and Illinois 25 (Liberty Street, northward; St. Charles Street, southward) as it crosses the Fox River. As the westbound traffic passes over it, the east end of the bypass has an eastern entrance for Villa Street (the original routing of US 20 in the city east of the Fox River).

After its interchange with Villa Street, the freeway reverts to a four-lane surface street, maintaining at least that width for the rest of its routing in Illinois.

Chicago suburbs

As Elgin enters Cook County, US 20 becomes Lake Street, serving as a major east-west arterial surface link from the western suburbs to Chicago. After exiting Elgin, Lake Street travels through Streamwood and Bartlett, where it has an overpass interchange with Illinois 59 (Sutton Road).