The youth was gored in the back while celebrating Saturday's Apostle James feast at Isso, 322 kilometers (200 miles) southeast of Madrid.
A police spokesman says he died hours later after two operations failed to repair massive liver injuries.
A 60-year-old man was also injured when he was gored in a leg.
Fiestas including bulls are common in Spain, and critics and aficionados alike often agree that security is lax at many small town bull runs.
That means almost anyone can participate, even if they're drunk or unfit.
The police spokesman spoke Sunday on condition of anonymity because he is not allowed to be cited by name.