Delaware County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 17,488. The county seat is Manchester. The county takes its name after the U.S. state of Delaware.

U.S. Highway 20 (US 20) in Iowa is a major east–west artery which runs across the state, separating the northern third of Iowa from the southern two-thirds. It enters Iowa from Nebraska, concurrent with Interstate 129 (I-129) and US 75, crossing the Missouri River at Sioux City. US 20 runs in a more-or-less straight line across Iowa, paralleling 42° 27' N. It leaves Iowa in Dubuque by crossing the Mississippi River into Illinois. Since October 2018, US 20 is a four-lane divided highway for its entire length in Iowa.