In a case criticised by rights groups, Nabeel Rajab, founder of the Bahrain Centre for Human Rights, had been serving three years for leading protests against the wide powers of the Sunni Muslim al-Khalifa dynasty which rules the island kingdom.
Bahrain, the base of the U.S. Navy's Fifth Fleet, has been in turmoil since the pro-democracy protests led by its Shi'ite Muslim majority erupted last year. Washington has called on its ally to talk to the opposition.
A hero to protesters but a villain for those Bahrainis who fear the protests will bring Shi'ite Islamists to power, Rajab was originally sentenced by a lower court in August, a verdict Washington said was deeply troubling and rights campaigners called a "dark day for justice".
The judge ruled in three cases on Tuesday, all related to participating in peaceful protests, and handed Rajab a one-year jail sentence in one case and six months each in the other two cases, said lawyer Mohammed al-Jishi.
He said Rajab had yet to decide whether to appeal again, adding: "It is a very harsh verdict."
"We were expecting the judge to issue one sentence for the three cases collectively, but he treated them as three separate cases and each had a separate sentence," Jishi told Reuters by telephone from Manama.
Tuesday's hearing was attended by monitors from rights groups and foreign diplomats, Jishi said.
The Bahrain government's Information Affairs Authority said in August the charges against Rajab had been related to violence. Public prosecutors had said Rajab's participation in marches and "provocation of his supporters" led to violence, including throwing petrol bombs and blocking roads.
Rajab has been in jail for about seven months for other charges, Jishi said.
Since April, the authorities have stepped up efforts to crack down on unrest. Activists cite an increased use of shotgun pellets, whose use officials have declined to confirm or deny.
In November, Interior Minister Sheikh Rashed bin Abdullah al-Khalifa said the Gulf Arab kingdom had temporarily banned all rallies and gatherings to ensure public safety and stability are restored.
Several activists have been jailed for organising or taking part in unlicensed anti-government protests.
Bahrain's ruling family used martial law and help from Gulf neighbours to put down last year's uprising, but unrest has resumed.
The opposition says that little progress has been made towards its demands for reforms including a parliament with full powers to legislate and form governments. Many Shi'ites complain of political and economic marginalisation, a charge the government denies.