The move, condemned by human rights groups, could lead to more unrest in the Arab state.
Bahrain has been in turmoil since a protest movement dominated by majority Shi'ite Muslims erupted last February during a wave of revolts against authoritarian Arab governments.
The Sunni Muslim ruling al Khalifa family put down the uprising with martial law, Saudi Arabian troops and police from the United Arab Emirates, but unrest has resumed, with near- daily clashes between Shi'ites and police.
The verdicts, originally issued by a military court against 21 men, including seven in their absence, include eight life sentences. Thirteen men remain in jail after one was released. Defence lawyers said the ruling could be appealed.
A prosecution official said six of the men were guilty of having "intelligence contact" with Iran and its Lebanese Shi'ite ally Hezbollah, seeking to overturn Bahrain's system of government and violating the constitution. Hezbollah denies involvement.
The men deny the charges, saying they wanted only reform. Four were acquitted of charges of foreign intelligence communications and seeking to change and overthrow the system of government, but their sentences remained the same.
Mohammed al Jishi, a defence lawyer, said this was partly due to other charges not among a total of 12 declared by the judge or media. The other charges included insulting the army, inciting sectarian hatred and organising protests without permission.
The authorities have initiated low-level talks with opposition groups, but said these groups must do more to stop street violence. The opposition says this stance is to avoid concessions and they are not responsible for clashes.
Amnesty's Middle East and North Africa Deputy Director Hassiba Hadj Sahraoui called the verdict "outrageous".
The men are viewed by some Bahrainis as popular heroes whose release could reinvigorate the protest movement, which demands parliamentary powers to legislate and form governments. Bahraini Shi'ites say they face discrimination, a charge the government denies.
Jane Kinninmont, of policy institute Chatham House, said the verdict sent a message to protesters and Western countries.
"The authorities may be trying to show their strength ahead of a planned dialogue with political societies," she said, saying this could backfire if clashes escalate.
Though US officials are keen for a release of prisoners to help restore calm, Washington has avoided irritating Manama with public calls over the case.
The presence of US warships helps ensure a free flow of oil exports from the Gulf, which Iran has threatened to blockade if the stand-off with Western powers over its nuclear energy programme turns into military confrontation.
Eight of the men received life sentences in a military court last year, including opposition leader Hassan Mushaimaa, who advocated turning Bahrain into a republic.
Ibrahim Sharif, leader of the opposition Waad party and the only Sunni in the group, is serving a five-year sentence while blogger Ali Abdulemam was given a 15-year term and is in hiding.
Prosecutors said the men got a fair trial. But Mr Jishi said the 13 in jail refused to attend in protest against the judge who closed previous sessions to the public when the men said they were abused in detention to force confessions.