The truck driver had parked along the side of the road in Betanzos in the northwest region of Galicia to inspect his cargo, when he caught sight of something unusual.
It was the body of a man, about 40 to 50 years old, hidden amid a grassy area along the highway, local newspaper La Voz de Galicia reported on Thursday.
Upon closer inspection, the body was missing an arm as well as most of its abdomen - and had become partially mummified.
The driver called police to the scene, who are now trying to determine by DNA tests who the man was. They estimate that he has been dead for about two months.
The mummified man had an incision on his head and investigators are awaiting the results of the autopsy to determine whether the death was an accident, or murder.
Natural mummification or preservation can occur when a body is exposed to cold temperatures, anaerobic conditions like in a bog, or dry heat.
The mummified body of a German sailor baffled authorities when it was found last weekend drifting in a half-submerged yacht on the Pacific ocean, the corpse preserved by salt water and sea air.