Ibrahim Sharif, whose secular Waed movement took part in 2011 protests alongside parties representing the Gulf state's Shia majority, had strongly denied the latter charge.
Jailed Bahrain opposition chief in trouble over tweets
But in a statement, the prosecutor general expressed disappointment with the decision of the High Criminal Court and raised the possibility of an appeal to the Court of Cassation on the more serious accusation.
Sharif already served four years of a five-year sentence handed down over the 2011 protests for an elected prime minister in the Sunni-ruled kingdom before being released under a royal amnesty last June.
But he was rearrested the following month after he spoke at a memorial service for one of those killed during the suppression of the month-long Arab Spring-inspired demonstrations in which he played a prominent role.
At least 89 people have been killed in clashes with security forces since 2011, while hundreds have been arrested and put on trial, human rights groups say.
Dozens of dissidents have since been jailed or stripped of their citizenship.
Bahrain to free US citizens accused of illegal gathering
Sheikh Ali Salman, leader of the main Shia opposition bloc Al-Wefaq, received a four-year sentence last June for inciting disobedience.
Tiny but strategic Bahrain, which is connected to Saudi Arabia by a causeway, lies across the Gulf from Shia Iran and is home base for the US Fifth Fleet.
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