The Avenue of the Saints is a 563-mile-long (906 km) highway in the Midwestern United States that connects St. Paul, Minnesota, and St. Louis, Missouri.

The Avenue of the Saints starts at exit 28A on Interstate 64 and US 40, which is also the location of the southern end of its concurrency with US 61, and runs west in the direction of Chesterfield, crossing I-270 shortly after leaving its southern terminus. The route turns northwest in Chesterfield, crosses the Missouri River via the Daniel Boone Bridge, and runs to Wentzville, where the route intersects with I-70, which is also the western terminus of I-64, and the northern terminus of the concurrency with US 40. The route continues north as just US 61 from this location, intersects with the western terminus of I-72 in Hannibal, and crosses the Des Moines River into Iowa, just after the route reaches the northern terminus of its concurrency with US 61 and is co-signed with Missouri Route 27 to the state line.

In Iowa, the Avenue of the Saints is a 282-mile-long (454 km) highway, which begins in Lee County where Missouri Route 27 crosses the Des Moines River, and ends at the Minnesota state line in Worth County, concurrent with Interstate 35. Construction of the Avenue of the Saints corridor in Iowa was completed on May 23, 2006.

The route runs north from the Missouri state line as just Iowa Highway 27 for about 10 miles, then is co-signed with US 218 until just south of Cedar Rapids, crossing I-80 in Iowa City. This is also the location of the southern terminus of its concurrency with I-380, and this concurrency terminates in Waterloo. Near Floyd, US 218 splits off, just after the route starts a concurrency with US 18, running west to its intersection with I-35, where the route turns north again and runs into Minnesota.

In 2001, the Iowa Department of Transportation designated the Avenue of the Saints as Iowa Highway 27. Prior to its creation, motorists wanting to travel the Avenue through Iowa had to follow a lengthy list of directions: (heading north from Missouri) Iowa Highway 394, U.S. Route 218, Interstate 380, U.S. Route 20, Iowa Highway 58, US 218 (again), U.S. Route 18 and Interstate 35.

In Minnesota, the Avenue of the Saints is co-signed with I-35 for the entirety of its length in that state. The route runs straight north, crosses I-90 in Albert Lea, and runs to its northern terminus in St. Paul, shortly after crossing the Mississippi River via the Lexington Bridge.

The Avenue of the Saints was the idea of Mount Pleasant, Iowa businessman Ernest Hayes, who envisioned a four-lane highway between St. Paul and St. Louis in the 1980s. The Iowa Department of Transportation decided to study the idea in 1988. Several politicians endorsed the idea, including Mount Pleasant mayor (and future Iowa governor) Tom Vilsack, Senator Charles Grassley of Iowa, and Congressmen David Nagle and Fred Grandy of Iowa and Dick Gephardt of Missouri.

By the end of 1989, four possible routes for the Avenue of the Saints were under consideration by the Federal Highway Administration. Two of the rejected routes would have followed U.S. Route 52 and U.S. Route 63 from St. Paul through Rochester, Minnesota, to Waterloo, Iowa. The third rejected route would have followed U.S. Route 61 from St. Paul through La Crosse, Wisconsin, and Dubuque, Iowa, to Davenport, Iowa, and U.S. Route 67 from Davenport crossing the Mississippi River through western Illinois (Forgottonia) to Alton, Illinois and crossing the Mississippi and Missouri Rivers to St. Louis.

In 1990 the FHWA chose its route for the Avenue of the Saints: The highway would follow the existing Interstate 35 from St. Paul to a point south of Clear Lake, Iowa; U.S. Route 18 to Charles City, Iowa; U.S. Route 218 to Cedar Falls, Iowa; Iowa Highway 58 and U.S. Route 20 around Cedar Falls and Waterloo, Iowa; Interstate 380 from Waterloo through Cedar Rapids to Interstate 80 near Coralville, Iowa and Iowa City, Iowa; U.S. 218 to Donnellson, Iowa; Iowa Highway 394 and Route B to Wayland, Missouri; and U.S. 61 and Interstate 64 from Wayland to St. Louis.

The Intermodal Surface Transportation Efficiency Act of 1991 made the Avenue of the Saints an official "high-priority corridor," and signs were put along the route by the end of the year. At that time the only four-lane segments were Interstates 35, 380, and 64; U.S. 20 around Waterloo; U.S. 218 from I-80 to Iowa Highway 22 near Riverside, Iowa; and two segments of U.S. 61 in Missouri (from La Grange to New London and from Bowling Green to St. Louis). The decision had also been made to build the Avenue of the Saints to expressway standards—with intersections at rural roads—rather than to full freeway standards as a cost-saving measure. Freeway segments would be built around cities that needed to be bypassed.

After the routing was approved, both Iowa and Missouri began constructing new four-lane segments. Iowa opened bypasses around Waverly (1998), Mason City (1999), Charles City (2000), Mount Pleasant (2001), and Donnellson (2004). A four-lane link between I-35 and I-380 was completed with the opening of a segment near Nashua in November 2003. Missouri completed four-lane segments from New London to Bowling Green in November 2000 and from Canton to La Grange in 2003.

In 2001 the Iowa Department of Transportation gave the Avenue of the Saints its own highway number: Iowa Highway 27. The number was added as an additional number to the existing routes; however, after the Donnellson bypass opened in 2004, Iowa 394 was decommissioned and Iowa 27 is now a standalone highway south of the split with U.S. 218.

On July 25, 2008, the final nine miles (14 km) of highway between the Lewis/Clark county line and Wayland, Missouri was open to four-lane traffic. A ceremony was held at the intersection of US 61 and Route 27 to commemorate the completion of the four-lane highway in Missouri. The Avenue of the Saints is now complete from suburban St. Louis to St. Paul (see Future construction below).

On December 8, 2004, a new four-lane bridge across the Des Moines River opened; this replaced an existing toll bridge operated by the Wayland Special Road District. A new four-lane road between the bridge and U.S. 61 south of Wayland also opened that day. This road was numbered Route 27 to match Iowa's number for the Avenue of the Saints.

In June 2005, a four-lane segment from the end of the Mount Pleasant bypass to the junction with Iowa Highway 16 east of Houghton was opened to four lanes of traffic. The segment of Iowa 27 between the split with U.S. 218 and the Des Moines River bridge opened to four lanes on August 25, 2005. The last remaining segment in Iowa was opened to traffic May 23, 2006.

The only portions of the road which do not bypass major towns are Waterloo, Iowa; Cedar Rapids, Iowa and Hannibal, Missouri.

A "festival" was held from June 16–18 amongst towns along the corridor to celebrate the completion of Iowa's segment.

As with many rural expressways in Iowa, the Avenue of the Saints has exit numbers in that state. The exit numbers correspond to the underlying U.S. Highway or Interstate Highway – U.S. 218, I-380, U.S. 20, U.S. 18 and I-35. The section along Iowa Highway 58 in Cedar Falls has no exit numbers, and the separate section south of Donnellson has one numbered exit—the northbound one to U.S. 218 south—based on Iowa 27's mileage.

On July 25, 2008, the Avenue of the Saints was completed with the opening of the final nine-mile (14 km) segment from the Lewis County line to Wayland in Clark County, Missouri.

Ongoing construction projects and future proposals along the Avenue of the Saints include: