Illinois Route 255, also referred to as the Alton Bypass, is a northwesterly extension of Interstate 255 in southwestern Illinois. Illinois 255 starts at I-270 in Pontoon Beach and ends at U.S. 67 north of Godfrey near its junction with Illinois Routes 267 and 111 (at a distance of approximately 23.3 miles (37.50 km)).

Illinois 255 is a four-lane, limited-access freeway for its entire length. It serves as an important circumferential artery for the northeastern portion of the St. Louis, Missouri metropolitan area. The final segment was completed on November 23, 2012 and runs 4.3 mi (7 km) from Seminary Road to U.S. 67 north of Godfrey near its junction with Illinois Routes 267 and 111. The highway passes just to the west of the St. Louis Regional Airport.

Although Illinois 255 was designed to federal interstate-highway standards, it was built by the Illinois Department of Transportation using state-provided funds.

Described as "The Road to Prosperity" by business leaders and government officials who initially proposed it in the late 1970s, Illinois 255 was part of a region-wide effort to create a high-speed alternative to U.S. Route 67 over the Clark Bridge through the city-center of Alton, and to two congested local routes (Illinois Routes 3 and 111) that roughly parallel the Illinois 255 corridor. Although it was part of the much larger Corridor 413 plan, the local politicians focused mostly on the segment from Interstate 270 to what was then Illinois Route 267 north of the Godfrey "Y" intersection. At that time, the Berm Highway, the Beltline extension, and the Madison Avenue connector were not built and both Illinois Routes 3 and 111 were over capacity between Alton and Interstate 270. In 1975, a six-month detour related with the closing of the Old Clark Bridge exposed the need for the Alton Bypass along with the other three routes. However, until 1985, the Alton Bypass was a low priority, as other local road projects in the area were considered more important.

The first 6.5 mi (10 km) segment of Illinois 255, from Interstate 270 to Illinois 143, was completed in October 1998 at the cost of $40 million (excluding land acquisition).

The second segment (a 7.2 mi (12 km) extension from IL-143 in Roxana to Fosterburg Road) was opened on October 20, 2006, approximately one year behind schedule, owing to the 2005 collapse of the Wisconsin-based construction company that held the contracts for that portion of the roadway. The second segment was completed at a cost of approximately $78.1 million.

The third segment of Illinois 255 from Fosterburg Road to Seminary Road in rural northeast Alton opened on August 22, 2008. Construction costs for this portion of the roadway totaled $25.1 million.

The fourth and penultimate segment of the highway from Seminary Road in Alton to Humbert Road in Godfrey opened on October 26, 2012.

The fifth and final segment of the highway runs 4.3 mi (7 km) from Seminary Road to U.S. 67 north of Godfrey near its junction with Illinois Routes 267 and 111 at a cost of $22 million.

Now complete, the IL-255 route may or may not be redesignated as an Interstate Route. If the corridor were to be signed as an extension of the current I-255 bypass, all exits north of I-270 would have to be renumbered. As a standalone spur route, IL-255 numbered as I-255 would violate the AASHO scheme for interstate numbering that requires an odd-number first digit (although as part of the larger I-255 bypass the route is not technically a spur even though it extends past the segment of I-55 that it bypasses). The entire corridor from Godfrey, IL to Mehlville, MO (comprising both I-255 and IL-255) could also carry secondary signage as U.S. 67 Bypass, since it would be a true bypass of US-67 (that passes through the St. Louis urban agglomeration) actually connecting with US-67 at both endpoints.

Construction of a new expressway route for US-67 between Jacksonville and Alton is planned to follow the corridor through White Hall, Carrollton, and Jerseyville. It will end in Godfrey near the Illinois 255 extension.

Information on future interchanges comes from the appendix ( to a corridor study done by The River Bend Growth Association (), the Chamber of Commerce, and economic development organization to communities north of St. Louis.

The entire route is in Madison County.

Control city from IL 111.

Northbound at IL 111/Exit 8.

Damaged signs off Exit 2, which has since been removed.