|Maintained by IDOT and ISTHA|
 (669.41 km)|
Length may include both directions of route where it is routed along one-way streets.
|South end||US 51 / US 60 / US 62 near Wickliffe, KY|
I-39 / I-90 at South Beloit
|North end||US 51 at the Wisconsin state line in South Beloit|
|Counties||Alexander, Pulaski, Union, Jackson, Perry, Washington, Jefferson, Marion, Fayette, Shelby, Christian, Macon, DeWitt, McLean, Woodford, Marshall, LaSalle, Lee, Ogle, Winnebago|
US 51 enters Illinois from Kentucky at the city town of Cairo, Illinois. The highway heads northbound to a village near Cairo called Mounds, and begins to overlap Interstate 57 (I-57), following it for 24 miles (39 km) to Dongola, before splitting and heading north. The highway remains two lanes wide from Dongola to Assumption with the exception of a 10-mile (16 km) section between Centralia and I-64.
Past Assumption, US 51 becomes an expressway to Decatur. In Decatur, US 51 follows I-72 to bypass the city. US 51 leaves I-72 after eight miles (13 km), and it heads north to Bloomington– Normal as an expressway. At Bloomington–Normal, US 51 follows I-74 for a mile, then I-55 for seven miles (11 km), before following I-39 for 140 miles (230 km).
US 51 follows I-39, intersecting I-80 and I-88 along the way. The highway also follows US 20 south of Rockford (while still following I-39). I-39/US-51 join I-90, making US 51 one of the few toll highway in Illinois that is a U.S. Highway. US 51 exits I-39/I-90 just a mile south of the Wisconsin state line. Then US 51 follows Illinois Route 75 (IL 75) west to the intersection of IL 251, and then turns north through South Beloit, Illinois to enter Wisconsin.
US 51 was established in Illinois in the 1920s. Over the years, it became a heavily traveled highway, often experiencing many accidents, particularly in the northern half of the state. This highway gained the moniker of "Killer 51". As a result of these problems, Illinois pushed for a new four lane highway along the corridor. One was proposed in the 1960s and 1970s as part of a proposed supplemental freeway system, but only the area between Rockford and Decatur was given a high priority. 
In the 1980s and early 1990s, US 51 was rebuilt to Interstate Highway standards between Rockford and Bloomington– Normal on a new alignment. This new highway became Interstate 39, and US 51 was rerouted onto it. In addition, north of Rockford, US 51 was rerouted onto the existing I-90 tollway segment between South Beloit and Cherry Valley. Most of the old highway became IL 251.
South of Bloomington–Normal to Decatur, US 51 remained largely on the original alignment, and it was expanded to an expressway. Further continuation of the expressway occurred in the 2000s, when the highway was upgraded southward from Decatur to Assumption, Illinois.
Plans are underway to continue the US 51 expressway south of Assumption to Centralia.  An environmental impact statement is currently being prepared. However, construction funding remains elusive.
|Alexander||Cairo||0.0||0.0||US 51 south / US 60 east / US 62 east – Wickliffe||US 51/US 60/US 62 continue into Kentucky|
|0.2||0.32||US 60 west / US 62 west – Charleston||Northern end of US 60/US 62 concurrency|
|4.5||7.2||IL 37 north – Mound City||Southern terminus of IL 37|
|5.3||8.5||1||I-57 south / IL 3 north – Olive Branch, Memphis||Southern end of I-57 concurrency; southern terminus of IL 3|
|28.8||46.3||25||I-57 north – Chicago||Northern end of I-57 concurrency; northbound exit and southbound entrance|
|Anna||36.0||57.9||IL 146 – Jonesboro, Vienna|
|Jackson||Carbondale||55.0||88.5||IL 13 – Murphysboro, Marion|
|De Soto||61.3||98.7||IL 149 – Murphysboro, Hurst|
|Perry||Du Quoin||72.4||116.5||IL 14 east – Christopher||Western terminus of IL 14|
|75.5||121.5||IL 152 west – Pyatts||Eastern terminus of IL 152|
|Sunfield||80.4||129.4||IL 154 – Red Bud, Sesser|
|Washington||Ashley||98.0||157.7||IL 15 west – East St. Louis||Western end of IL 15 concurrency|
|100.9||162.4||IL 15 east – Mount Vernon||Eastern end of IL 15 concurrency|
|Richview||105.7||170.1||I-64 – St. Louis, Louisville||Exit 61 on I-64|
|Irvington||109.6||176.4||IL 177 west – Belleville||Western terminus of IL 177|
|Marion||Centralia||116.2||187.0||IL 161 – New Baden, Kell|
|Sandoval||122.7||197.5||US 50 west – Carlyle||Western end of US 50 concurrency|
|123.0||197.9||US 50 east – Salem||Eastern end of US 50 concurrency|
|Fayette||Vandalia||146.9||236.4||US 40 east / IL 185 east – Bluff City||Eastern end of US 40/IL 185 concurrency|
|148.5||239.0||US 40 west / IL 185 west – Taylor Springs, Mulberry Grove||Western end of US 40/IL 185 concurrency|
|148.7||239.3||I-70 – St. Louis, Indianapolis||Exit 63 on I-70|
|Christian||Pana||178.1||286.6||IL 16 west||Western end of IL 16 concurrency|
|178.6||287.4||IL 29 north||Southern terminus of IL 29|
|||180.2||290.0||IL 16 east||Eastern end of IL 16 concurrency|
US 51 Bus. north
|Harristown||214.7||345.5||133||I-72 west / US 36 – Springfield, Tuscola||Southern end of I-72 concurrency; signed as exits 133A (east) and 133B (south)|
|Decatur||219.2||352.8||138||IL 121 – Decatur, Lincoln|
I-72 east / US 51 Bus. south – Champaign, Urbana, Decatur
|Northern end of I-72 concurrency|
US 51 Bus. north – Clinton
US 51 Bus. south – Clinton
|McLean||Heyworth||251.9||405.4||US 136 – McLean, Rantoul|
I-74 east / US 51 Bus. north (Main Street) – Indianapolis, Bloomington
|Southern end of I-74 concurrency|
|||262.2||422.0||157A||I-55 south – St. Louis||Southern end of I-55 concurrency|
|Bloomington||262.4||422.3||157B||I-55 Bus. north (Veterans Parkway)||Signed as exit 134B (north) southbound|
|265.5||427.3||160||US 150 / IL 9 (Market Street) – Pekin|
|||269.1||433.1||163||I-74 west – Peoria||Northern end of I-74 concurrency|
|||270.3||435.0||164||I-39 north / I-55 north – Chicago||Northern end of I-55 concurrency; southern end of I-39 concurrency; southern terminus of I-39|
US 51 Bus. south – Bloomington, Normal
|||278.3||447.9||8||IL 251 (Lake Bloomington Road)|
|Woodford||El Paso||284.9||458.5||14||US 24 – El Paso, Peoria|
|Woodford||292.9||471.4||22||IL 116 – Peoria, Pontiac|
|Marshall||Wenona||305.6||491.8||35||IL 17 – Lacon, Wenona|
|LaSalle||Lostant||312.2||502.4||41||IL 18 – Henry, Streator|
|||322.3||518.7||51||IL 71 – Hennepin, Oglesby|
|||323.0||519.8||52||IL 251 – Peru, LaSalle|
|Illinois River||326.5||525.5||Abraham Lincoln Memorial Bridge|
|La Salle||327.3||526.7||57||US 6 – LaSalle, Peru, Ottawa|
|||329.6||530.4||59||I-80 – LaSalle, Peru, Chicago, Des Moines||Signed as exits 59A (east) and 59B (west)|
|Troy Grove||336.8||542.0||66||US 52 – Troy Grove|
|Mendota||342.7||551.5||72||US 34 – Mendota, Earlville|
|Lee||Paw Paw||353.1||568.3||82||Paw Paw|
|||357.9||576.0||87||US 30 – Sterling, Rock Falls, Aurora|
|Ogle||Rochelle||368.0||592.2||97||I-88 / IL 110 ( Ronald Reagan Memorial Tollway) – Moline, Rock Island, Chicago||Signed as exits 97A (east) and 97B (west)|
|370.1||595.6||99||IL 38 – DeKalb, Rochelle|
|||375.1||603.7||104||IL 64 – Sycamore, Oregon|
|Monroe Center||381.6||614.1||111||IL 72 – Genoa, Byron|
|Rockford||390.2||628.0||US 20 west – Freeport, Rockford||Southern end of US 20 concurrency, I-39/US 51 southbound exits US 20 via exit 97|
|Cherry Valley||392.7||632.0||122A||US 20 east – Belvidere||Northern end of US 20 concurrency|
|393.4||633.1||17||I-90 east ( Jane Addams Memorial Tollway) – Chicago||Southern end of I-90 concurrency; I-39/US 51 uses I-90's exit numbers from here north|
US 20 Bus. (State Street)
|Last free exit northbound|
|398.4||641.2||12||Riverside Boulevard||Toll on northbound exit and southbound entrance|
|Machesney Park||401.9||646.8||9||IL 173 (West Lane Road)||Toll on northbound exit and southbound entrance|
|Rockton||407.3||655.5||South Beloit Toll Plaza|
|South Beloit||408.1||656.8||3||Rockton Road (CR 9)||Last free exit southbound|
|414.0||666.3||1||I-39 north / I-90 west / IL 75 east – South Beloit, Madison||Northern end of I-39/I-90 concurrency; eastern end of IL 75 concurrency|
|415.6||668.8||IL 75 west / IL 251 south – Freeport, Rockford||Western end of IL 75 concurrency; northern terminus of IL 251|
|415.95||669.41||US 51 north – Beloit||Continuation into Wisconsin|
|1.000 mi = 1.609 km; 1.000 km = 0.621 mi|
- Illinois Technology Transfer Center (2007). "T2 GIS Data". Illinois Department of Transportation. Retrieved November 8, 2007.
- "Illinois Supplemental Freeway system proposal" (PDF). Retrieved February 17, 2015.
- "US 51 Environmental Impact Statement". Illinois Department of Transportation. Retrieved February 17, 2015.
- Google (March 19, 2017). "Overview Map of US 51" (Map). Google Maps. Google. Retrieved March 19, 2017.
- Media related to U.S. Route 51 in Illinois at Wikimedia Commons
|U.S. Route 51|