Sheriff's spokesman Maj. John Clark said the deputy responded to a report of wrecked pickup truck about 4 a.m. Monday on the island about 40 miles southwest of Charleston.
Clark said the deputy saw a bloodied man standing in the back of the truck and saw a gun holster in the cab.
The deputy asked to see the man's hands and when he didn't immediately respond, the deputy took cover behind his patrol cruiser.
Clark said the man then pulled out the gun and shot himself in the chest. He died at the scene. His identity was not immediately released.