Nabeel Rajab, a leading figure in pro-democracy protests that swept Bahrain in 2011, was already serving a two-year sentence over a news interview in which he said Bahrain tortured political prisoners.
The new convictions were for “insulting a neighbouring country” and “insulting national institutions” in comments posted on Twitter, activists said.
The Bahraini Embassy in London said the prosecution and Rajab’s defence team had had ample opportunity to present their cases.
“The trials for the cases were overseen by numerous observers, including representatives from foreign embassies and NGOs,” the embassy said in a statement.
“Bahrain continues to make it clear that both cases – or indeed any other criminal cases within Bahrain – do not relate to political views or political expression,” it added.
Sayed Ahmed Alwadaei, an activist with the London-based Bahrain Institute for Rights and Democracy (BIRD), said Bahraini courts were barring citizens from criticising the authorities.
Bahrain has cracked down on perceived threats since Arab Spring protests in 2011 led mainly by majority Shi’ite Muslims, were quashed with help from Gulf Arab neighbours.
The Sunni Muslim-led monarchy has closed two main political groups – the Shi’ite Islamic al-Wefaq and the secular Waad -, revoked the citizenship of the top Shi’ite cleric and banned activists from travel and put some on trial.
Bahraini newspapers reported on Wednesday that parliament has passed a legislative amendment that would bar members of the two dissolved groups from competing in upcoming parliamentary election expected in October or in November.
Authorities accuse Iran, the region’s majority Shi’ite power, of being behind several bomb attacks on its security forces since 2011, something Iran denies.
Bahrain hosts the U.S. Fifth Fleet, a key naval base in the oil exporting region riven by animosity between Bahrain’s main ally, Saudi Arabia, and Iran.
A U.S. embassy representative attended an earlier hearing for Rajab, State Department spokeswoman Heather Nauert said on Tuesday, adding that Washington was “disappointed” his earlier verdict had been upheld.
“He’s a prominent human rights activist … we continue to have conversations with the government of Bahrain about our very serious concerns about this,” Nauert said.
(Reporting by Noah Browning, Stephanie Nebehay in Geneva and Angus MacSwan in London; Editing by Robin Pomeroy, Richard Balmforth, William Maclean)