"Murtaja Qureiris, detained since 2014 when he was only 13, has been released following the expiry of his sentence," ALQST for Human Rights said on Twitter.
"Qureiris had been at risk of the death penalty, and was later sentenced to eight years on charges including participating in protests at the age of 10," the London-based group added.
It said he "should never have been arrested".
Amnesty International in the Gulf said it was "happy" for Qureiris and his family.
Saudi Arabia has released several young Shia men in recent months, including Ali al-Nimr, who was arrested as a minor in 2012 for taking part in anti-government protests and received a death sentence that was later commuted.
Nimr is a nephew of Shia cleric Nimr al-Nimr, who was executed by Saudi Arabia in 2016, leading to the severance of diplomatic relations between Riyadh and Tehran.
Saudi Arabia in recent years has announced a series of reforms to sentencing by the courts, including a moratorium on the death penalty for drug offences, as well as abolishing court-ordered floggings.
In April 2020, the kingdom announced it was ending the death penalty for those convicted of crimes committed when they were under 18.
*This story was edited by Ahram Online