Pompeys Pillar is a Census-designated place in Yellowstone County, Montana, United States.

The town of Pompeys Pillar was founded in 1907 and was named after and situated less than a mile east of Pompeys Pillar National Monument, a 150 foot tall sedimentary rock formation best known for William Clark's inscription of his name and the date July 25, 1806 on its surface.

The Northern Pacific Railroad connected the town to Billings, about 29 miles southwest.

Pompeys Pillar National Monument was created in 2001 and placed under the jurisdiction of the Bureau of Land Management.

In 2006 a new visitor center and museum complex opened, coinciding with the bicentennial of the Lewis and Clark Expedition.