Rajab, a leading figure in 2011 protests against the Gulf state's Sunni minority rulers, was already serving a two-year sentence handed down last July for "disseminating rumours and false information" in television interviews critical of the government.
He has served multiple stints in prison since 2012, all linked to his role in the protests.
In the latest case, Rajab was found guilty of insulting a neighbouring country and spreading false news and rumours, a judicial source told AFP on condition of anonymity.
The charges also relate to tweets critical of Bahrain's treatment of inmates at the notorious Jaw prison, south of the capital Manama.
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The kingdom's leading human rights groups, the Bahrain Center for Human Rights and the Bahrain Institute for Rights and Democracy, confirmed the charges and sentence.
Human rights groups warned last month they feared for the health of Rajab, who has been hospitalised multiple times in recent years.
Bahrain, a tiny Shiite-majority kingdom strategically located between Saudi Arabia and Iran, is home to the US Navy's Fifth Fleet and a British military base that is under construction.
Since 2011, authorities have jailed dozens of high-profile activists and disbanded both religious and secular opposition groups.
They have stripped hundreds of those convicted of their citizenship, making many stateless, according to Amnesty International.