Clay County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 16,384. Its county seat is Spencer. Its name is in honor of Henry Clay Jr., a colonel who died in action in the Mexican–American War, and son of Henry Clay, famous American statesman from Kentucky.

Henry Clay Jr. was an American politician and soldier from Kentucky, the third son of US Senator and Representative Henry Clay and Lucretia Hart Clay. He was elected to the Kentucky House of Representatives in 1835 and served one term. A graduate of West Point, he served in the Mexican–American War and was killed in 1847 at the Battle of Buena Vista.