Serpent Mound, Ancient effigy mound in Adams County, United States.

Serpent Mound is an earthwork shaped like a snake in Adams County, United States, stretching over 1,300 feet across open land. The raised line winds across the landscape and ends in an oval form representing the head.

Communities living over 2,000 years ago shaped the mound from earth and stone into its current form. Later groups altered parts of the structure and adapted it to their own customs.

Squier and Davis read the open mouth as a serpent about to swallow a 120-foot oval they took for an egg; archaeologist J.P. McLean instead identified the body as a frog. The oval lies east of the head, and the tail ends in a triple coil. The site is on UNESCO's tentative list since 2008.

Trails loop around the raised form and offer views from different angles, while a tower gives a view from above. A visit usually takes one to two hours, with a chance to walk through the nearby grounds.

The mound stands on the Serpent Mound crater, an impact structure about 8.7 miles (14 km) across formed 252–330 million years ago. Geologists confirmed the impact origin in 2003, and erosion has erased the crater's surface expression, so no rim or depression is visible today.