K-18 is a primarily east–west state highway running between US-24 and I-70 in north central Kansas.

K-18 begins near the town of Bogue in Graham County as it branches off to the south from US-24. The highway then stairsteps to the southeast through the towns of Damar, Palco, Plainville, and Codell in Rooks County; Natoma in Osborne County; Paradise, Waldo, Luray, and Lucas in Russell County (K-18 is duplexed with US-281 for 9 miles (14 km) west of Luray); and Sylvan Grove, Lincoln, and Beverly in Lincoln County. As K-18 enters Ottawa County, it takes a due east bearing, traveling through Tescott and meeting US-81 north of Salina. The road briefly jogs to the north for around 2 miles (3.2 km) before turning east again to travel through Bennington and Talmage in Dickinson County and into Junction City in Geary County. In Junction City, K-18 meets US-77 and travels south along US-77 through the city until it hits I-70. K-18 then travels to the northeast along I-70 for eight miles (13 km) before exiting and continuing to the northeast toward Manhattan. K-18 crosses the Kansas River at the Riley County line and travels through Ogden into Manhattan. Once traveling through Manhattan, K-18 again crosses the Kansas River and parallels the river to the south into Wabaunsee County before ending at K-99 south of Wamego.