Iowa Highway 38 is a 98-mile-long (158 km) state highway that runs through eastern Iowa. Iowa 38 begins at Iowa Highway 92 in Muscatine and ends at Iowa Highway 3 near Greeley.

Iowa Highway 38 begins at the intersection of Iowa Highway 92 and Second Street, which is Iowa 92's last intersection before crossing the Mississippi River into Illinois. Just a few blocks away is the southern intersection with Iowa Highway 22, from which Iowa 22 heads towards Davenport. Iowa 22 overlaps Iowa 38 1.2 miles (1.9 km) until the intersection of U.S. Route 61. From its southern terminus to US 61, Iowa 38 is overlapped by U.S. Route 61 Business. From Muscatine, Iowa 38 continues north 8 miles (13 km) to the southern intersection with U.S. Route 6 south of Wilton. At Fifth Street in Wilton, US 6/IA 38 intersect a former alignment of US 6, which from 1980–2003 was known as Iowa Highway 927.

Just north of Wilton, U.S. Route 6 and Iowa Highway 38 cross into Cedar County. Shortly thereafter, US 6/IA 38 intersect Interstate 80 and split in opposite directions; US 6 splits to the east and IA 38 to the west. Iowa 38 overlaps I-80 from exit 267 to exit 271. From I-80, Iowa 38 continues north towards Tipton where it meets Iowa Highway 130 at its western end. North of Tipton, Iowa 38 overlaps U.S Route 30 for 1 mile (1.6 km), ending in Stanwood where Iowa 38 continues north.

Iowa Highway 38 enters Jones County south of Olin and crosses the Wapsipinicon River on Olin's northern edge. 4 miles (6.4 km) later, Iowa 38 intersects Iowa Highway 64, which Iowa 38 overlaps for 2 miles (3.2 km) before heading north towards Monticello passing through Center Junction and Scotch Grove. Next to the Monticello Regional Airport, Iowa 38 meets U.S. Route 151. 3⁄4 mi (1.21 km) later, Iowa 38 meets and overlaps the former alignment of US 151, now U.S. Route 151 Business. At First Street, it leaves US 151 Bus. to the west for a few blocks before turning north again towards Hopkinton and crossing the Maquoketa River.

Just north of Sand Springs, Iowa Highway 38 intersects Delaware County Route X47, which is a cutoff to eastbound U.S. Route 20. From Hopkinton to Delhi, Iowa 38 is roughly parallel to the Maquoketa River. At Delaware, Iowa 38 intersects U.S. Route 20. Iowa 38 briefly overlaps US 20's former routing, now Delaware County Route D22, north of Delaware. Iowa 38 continues north through Greeley and ends 3 miles (4.8 km) north of Greeley at Iowa Highway 3.