According to law enforcement officials, the Indy 500 racer was arrested after he was identified as the driver of a car that sideswiped another on the Las Vegas Beltway early Thursday morning.
Unser's car was practically unscathed in the incident, but the other car crashed into a cement center median. No injuries were reported at the scene.
Unser, who won the Indy 500 in both 1992 and 1994, underwent treatment for alcohol abuse in 2002.