0621 GMT March 22, 2019
Sheikh Ali Salman, who headed the now-banned Al-Wefaq movement, and two of his aides had been acquitted by the high criminal court in June, a verdict the public prosecution appealed, AFP reported.
The public prosecutor said in a statement that the three had been unanimously sentenced by the appeals court for "acts of hostility" against Bahrain and "communicating with Qatari officials... to overthrow constitutional order".
Bahrain, along with Saudi Arabia and the United Arab Emirates, severed all ties with Qatar in 2017, banning their citizens from travel to or communication with the emirate over its alleged ties to both Iran and radical groups.
Sunday's verdict against the charismatic Shia cleric can still be appealed.
Ruled for more than two centuries by the Al-Khalifa dynasty, Bahrain has been hit by waves of unrest since 2011, when security forces crushed protests demanding a constitutional monarchy and an elected prime minister.
Opposition movements, both religious and secular, have been outlawed and hundreds of dissidents have been imprisoned – many of them stripped of their citizenship in the process.
Salman's Al-Wefaq was dissolved by court order in 2016.
Another opposition group, the leftist National Democratic Action Society, or Al-Waad, was banned the following year over allegations of links to terrorists.
Salman is currently serving a four-year sentence in a separate case – "inciting hatred" in the kingdom.
Human rights groups have said cases against activists in Bahrain – men and women, religious and secular – fail to meet the basic standards of fair trials.
Advocacy groups like Amnesty International slammed Sunday's ruling against the 53-year-old Salman and his aides, Hassan Sultan and Ali al-Aswad as political reprisal.
"This verdict is a travesty of justice that demonstrates the Bahraini authorities' relentless and unlawful efforts to silence any form of dissent," said Heba Morayef, Amnesty's Middle East and North Africa director.
"Sheikh Ali Salman is a prisoner of conscience who is being held solely for peacefully exercising his right to freedom of expression."
Bahraini activists said Sunday's verdict highlighted a broader regional policy toward freedom – the same policy that saw Saudi Arabian journalist Jamal Khashoggi murdered this month.
The verdict comes ahead of controversial parliamentary elections that Bahrain's King Hamad has called for November 24.
Dissolved opposition parties, including Al-Wefaq and the secular Al-Waad, do not have the right to put forward their own candidates in the vote.
Bahrain, a key ally of the United States and home to the US Fifth Fleet, last year ratified an amendment to the constitution granting military courts the authority to try civilians charged with terrorism, a term loosely defined by the country's penal code.
In June, Bahrain amended its law on political rights, prohibiting "leaders and members of political associations dissolved for violating the kingdom's constitution or its laws" from running in legislative elections.
The United Nations and rights groups including Amnesty International and Human Rights Watch have criticized the Bahraini monarchy over its treatment of protesters.
Amnesty and HRW categories Salman and other jailed opposition leaders prisoners of conscience.