The youth was gored in the back while celebrating yesterday's Apostle James feast at Isso, 322 km 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.
