Spain's Supreme Court also ruled on Tuesday that the defendant, who was not named, should be suspended from his position.
The officer was part of a tactical unit that was ordered to arrest two alleged extortionists in 2008, TheLocal.es reported.
The squadron's command officer gave his men the arrest order as the suspects were entering the vehicle, and authorized the use of stun grenades.
When the suspects refused to get out of the vehicle, the defendant threw his stun grenade on the 25-year-old victim's lap.
The detonation caused the victim to lose most of his testicles, and he will now be infertile for life, TheLocal.es reported.