Videos posted on social media showed what appeared to be a fire or explosion in a building near the base of the dam.
The city of Boulder City posted on its Twitter account that the fire was extinguished before the fire department arrived.
Hoover Dam is located in the Black Canyon of the Colorado River on the border between Nevada and Arizona.
According to the federal Bureau of Reclamation, the dam is 726 feet tall and 660 feet at its base, as long as two football fields measured end-to-end.
As many as 20,000 vehicles a day drive across the 45-foot wide top of the dam, which is a National Historic Landmark.
