Sterne finished at 18-under-par 266 at Madrid's Club de Campo.
Anders Hansen was second at 268 and Terry Price took third, another shot back.
The 23-year-old Sterne needed a strong finish to keep his tour card for 2005.
"The whole year has been a tough year, especially the last three weeks," Sterne said. " "I knew I had to perform. I wanted to give myself a chance going into the back nine, and I did and I pulled it off."