"Mahmud Mohsen succumbed to his wounds after being hit by buckshot on Wednesday in Sitra," Al-Wefaq said on Twitter.
The interior ministry said police had been notified that a dead person was admitted to a clinic in Sitra, and added that an investigation was underway.
Witnesses said the clashes broke out in the afternoon during a ceremony to mark the death of a Shiite killed in an explosion on Friday last week.
Bahrain remains deeply divided three years after security forces crushed an Arab Spring-inspired uprising, with persistent protests sparking clashes with police, scores of Shiites jailed on "terror" charges and reconciliation talks deadlocked.