In the U.S. state of Illinois, U.S. Route 20 (abbreviated US 20) is a major arterial highway that runs from the Iowa state line at East Dubuque at the northwestern tip of Illinois, to the Indiana state line at Chicago south of the Chicago Skyway. A distance of 233.93 miles (376.47 km).

For its entire length, US 20 is designated as the General Ulysses S. Grant Highway, often abbreviated the U.S. Grant Highway. However, this name is only commonly used west of Elgin. From Elgin to Melrose Park, US 20 is commonly called Lake Street. It then is called Mannheim Road, and further south La Grange Road. US 20 is then called 95th Street as it turns east through Chicago's southwestern suburbs. It also takes on the names of several smaller streets through the southern portion of Chicago, before leaving Illinois on Indianapolis Boulevard.

As the highway enters Illinois across the Julien Dubuque Bridge, it continues southeasterly for a while, paralleling the Upper Mississippi River before turning east into the rugged country of the Driftless Area in Jo Daviess County, then turning flat and straight west of Stockton, Illinois and entering Stephenson County. The Ulysses S. Grant Home, a national landmark, fronts near the highway at the eastern edge of Galena, Illinois.

In Freeport, US 20 is routed onto a limited-access freeway bypass. Between Freeport and Rockford there are two traffic light-controlled intersections. The Rockford bypass routes US 20 onto a full limited-access freeway; it remains this way until it overlaps Interstate 39 southeast of downtown Rockford. Interchanges on the Rockford Bypass include West State Street (Business US 20), South Meridian Road, Montague Road, South Main Street (Illinois Route 2), 11th Street (Illinois Route 251), and South Alpine Road.

East of Rockford, the routing of US 20 runs parallel with Interstate 90, with US 20 serving minor cities that are not accessible from the Jane Addams Memorial Tollway. It is a four-lane divided highway from I-39 to just east of Belvidere; for the rest of this segment, it is a winding, rural two-lane road.

US 20 travels through the downtowns of the small towns and unincorporated areas of Garden Prairie, Harmony and Starks, as well as the city of Marengo. At Starks, eastbound US 20 overlaps southbound Illinois Route 47 and westbound Illinois Route 72, forming a wrong-way concurrency. All three routes pass beneath a railroad viaduct before separating south of Starks.

Continuing east, US 20 travels through Pingree Grove before entering the Elgin area. Before entering Elgin, US 20 widens to a four lane limited-access freeway that bypasses Elgin. This road is known as the Elgin Bypass.

At the end of the Elgin bypass, US 20 takes on the name of Lake Street. US 20 has numerous high-volume intersections, including a grade-separated interchange with Illinois Route 59. In Hanover Park, an early-2000s reconstruction project widened lanes on the road and created a median for safer travel; east of Barrington Road / County Farm Road, the road briefly becomes six lanes for one mile (1.6 km) until the Elgin-O'Hare Expressway.

US 20 continues in an east-southeast direction through the town center of Bloomingdale and becomes six lanes at the Bloomingdale-Roselle border. It maintains six lanes through a very high traffic corridor to Interstate 290 and onwards to Illinois Route 53 before it returns to four lanes while serving downtown Addison, paralleling Interstate 290 to the north. Past Elmhurst, the road overlaps Interstate 290 for a mile, technically giving US 20 ten lanes of traffic, although the rightmost two are exit-only lanes. At the interchange, westbound US 20 has only one lane, but eastbound US 20 has two.

US 20 continues through Stone Park and Melrose Park before turning south and joining U.S. Routes 12/45. At this point, US 20 becomes Mannheim Road, and later, La Grange Road, until it turns onto 95th Street eastbound with U.S. 12. This combination later joins with U.S. Route 41, where the three routes follow Ewing Avenue and Indianapolis Boulevard, which are the last portions of US 20 prior to its crossing the state line into Indiana.

US 20 has been rerouted around the cities of Freeport, Rockford, Elgin and Chicago. A bypass is currently planned for Galena; see Future.

The longest Business U.S. Route 20 in Illinois runs 21 miles (34 km) from just west of Rockford to just east of Belvidere. The route begins just west of Weldon Road and runs as State Street into downtown Rockford, where it is split into two one-way streets (Chestnut Street and Walnut Street for eastbound traffic, Jefferson Street for westbound traffic). The route continues east along State Street all the way through Rockford and becomes a two-lane highway between Rockford and Belvidere. Business 20 is then routed in an outer loop around the city of Belvidere before merging back with US 20 near Genoa Road and Interstate 90 (Northwest Tollway).

Business U.S. Route 20 in Elgin was removed in 1984. This ran along Larkin Avenue, Highland Avenue, Chicago Street and Villa Avenue. The current routing of US 20 south of Elgin is called the Elgin Bypass.

Business U.S. Route 20 in Chicago was removed in 1968. This existed from 1960 through 1968; before then, it was City U.S. Route 20 from 1938 to 1960.

The routing of US 20 has not changed since 1938. Prior to 1938, US 20 continued east on Lake Street east of Mannheim Road (U.S. Routes 12/45). It ran through downtown Chicago and exited Chicago south on what is now Torrence Avenue. In 1955, the Illinois General Assembly designated the entire length of US 20 in Illinois the "U.S. Grant Memorial Highway". New signs were produced in late 2007 to reflect this designation along its entire length.

The Illinois Department of Transportation is in the middle of a multiyear project for construction of a bypass for the cities of Galena and Freeport. Long-range plans call for a full four-lane highway from Galena to Freeport. Although the bypass has been discussed for decades, the project only began to receive funding in the early 2000s. As the only major corridor between Interstates 88 and 90, the Freeport to Galena project is part of a larger project to improve the viability of U.S. Route 20 between Waterloo, Iowa and Rockford.

Eventually, very long-range plans call for U.S Route 20 to be upgraded to a full four-lane freeway from Rockford to East Dubuque.