Clay County is the name of 18 counties in the United States of America:
- Named for Henry Clay, American statesman:
- Clay County, Alabama
- Clay County, Florida
- Clay County, Georgia
- Clay County, Illinois
- Clay County, Indiana
- Clay County, Kansas
- Clay County, Minnesota
- Clay County, Mississippi
- Clay County, Missouri
- Clay County, Nebraska
- Clay County, North Carolina
- Clay County, South Dakota
- Clay County, Tennessee
- Clay County, Texas
- Clay County, West Virginia
- Other counties:
- Clay County, Arkansas (named for John Clayton, and originally named Clayton County)
- Clay County, Iowa (named for Henry Clay, Jr., son of the statesman Henry Clay and a soldier in the Mexican War)
- Clay County, Kentucky (named for Green Clay, first cousin once removed of the statesman Henry Clay, a member of the Kentucky state legislature.)