U.S. Route 166

US-166's western terminus in South Haven, Kansas

US 166's eastern terminus approaching I-44 in Newton County, Missouri

US 166 is an east–west highway that meanders about 164 miles (264 km) along the Kansas–Oklahoma state line. The highway's western terminus is in South Haven, Kansas at an intersection with US-81, approximately 1-mile (1.6 km) north of the northern terminus of US-177. US 166's eastern terminus, which is also shared by US-400, is an intersection with I-44 just inside the Missouri state line, near the point where Kansas, Oklahoma, and Missouri meet, southwest of Joplin, Missouri near Loma Linda, MO.

About 4 miles (6.4 km) east from its intersection with US-81, the highway intersects with the Kansas Turnpike (I-35) at exit 4. Continuing east about 60 miles (97 km), US-166 crosses the Arkansas River into Arkansas City, where it runs concurrently with US-77 for approximately 2 miles (3.2 km), splitting off near Parkerfield.

Traveling 50 miles (80 km) east of Arkansas City, US-166 turns northeast, arcing above Cowley County Lake Dam and Cedar Vale. US. 166 crosses the Caney River approximately 0.5 miles (0.80 km) east of Chautauqua County Road 2. It turns south of Sedan approximately 0.6 miles (0.97 km) east of Chautauqua County Road 14. A business loop connects the main US-166 route with Sedan.

Continuing east from Sedan, about 34 miles (55 km) to Coffeyville, US-166 skims through Peru and Niotaze, US 166 crosses the Little Caney River 1.1 miles (1.77 km) to the east of Niotaze and proceeds east to the junction of US-75 near Havana. The two routes turn south for a 3-mile (4.8 km) concurrency. US-166 splits off just north of Caney turning east again through Tyro to Coffeyville. US-166 has a brief concurrency with US-169 beginning at the intersection of Walnut/Paterson Streets and Eleventh Street on the south side of downtown Coffeyville. The highway proceeds east on Eleventh Street, curves to the northeast and becomes Northeast Street. It then proceeds northeasterly past Walter Johnson Park and the Montgomery County Fairgrounds before it crosses the Verdigris River. US-166 will exit from US-169 0.4 miles (0.64 km) east of the Verdigris River and proceeds east toward the town of Chetopa

US-166 runs eastward about 50 miles (80 km) from Coffeyville bypassing Edna and Bartlett. US-166 passes through Chetopa, where US-166 and US-59 travel concurrently from 0.2 miles (0.32 km) east of the BNSF Railroad overpass, and proceeds south on Eleventh Street, then turns to the east near the Chetopa ball field. US-166/US-59 will run concurrently along Maple Street into downtown Chetopa. US-59 will then break off on Third Street and proceeds south towards the Kansas-Oklahoma state linesouth of the intersection. Leaving Chetopa, US-166 crosses over the Neosho River and crosses US-69 Alternate, 6 miles (9.7 km) before Baxter Springs.

US-166 and US-400 at the terminus of K-26

Heading east from Baxter Springs, US-166 crosses the Spring River. Approximately 2 miles (3.2 km) later, US-166 intersects with US-400. US-166/US-400 then turn sharply southeast for 6 miles (9.7 km) to cross the Missouri state line approximately ¹⁄₂ mile (0.80 km) from the I-44 interchange.

US 166's eastern terminus is its interchange with I-44, 0.6 miles (0.97 km) from the state line[1] and about 1,000 feet (300 m) from the Oklahoma–Kansas–Missouri tripoint.