They were prosecuted for lowering a flag from a school during demonstrations in Eastern Province and replacing it with a black Shiite banner.
The same source said the five replaced an official plaque on which was written "The Royal Kingdom of Saudi Arabia" with one stating "The Kingdom of Qatif of Tomorrow".
Demonstrations in Eastern Province, where most of the mainly Sunni kingdom's two million Shiites live, erupted in 2011 alongside a Shiite-led protest movement in neighbouring Bahrain.
Tensions eased later that year when seven senior Shiite dignitaries from Qatif responded favourably to a call from King Abdullah for the creation of a centre for dialogue between Sunnis and Shiites.