The United States, which has a major naval base in the country, has expressed concern about the case of Nabeel Rajab, a leading figure in pro-democracy protests that swept Bahrain in 2011, who 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.
There was no immediate comment from the Bahraini government.
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.
"Instead of rewarding Nabeel Rajab for his brave and commendable exposure of human rights abuses and advocacy for peace, the authorities have chosen to punish the messenger."
International rights groups denounced the ruling as a violation of freedom of expression.
"They are basically trying to clamp down on every kind of dissent. They don't seem to tolerate any criticism," Hanan Salah, senior researcher on Bahrain for Human Rights Watch, told Reuters in Geneva.
Amnesty International's regional director for the Middle East and North Africa Heba Morayef, said: "It is absolutely outrageous that he (Rajab) be forced to spend a further five years in jail simply for daring to voice his opinions online."
Bahrain has cracked down on perceived threats since the 2011 protests inspired by the "Arab Spring", led mainly by members of its majority Shi'ite population, 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 years of militant bomb and gun attacks on its security forces, something Iran denies.
Bahrain hosts the US Fifth Fleet, a key naval base in the oil exporting region riven by animosity between Bahrain's main ally, Saudi Arabia, and Iran.
A US 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.
Rajab's son Adam tweeted on Wednesday that, on hearing the verdict, his father laughed in the courtroom and flashed a peace sign.