He had suggested in a tweet that residents of an area in the capital known for its strong pro-government stance had supported the long-standing prime minister in return for unspecified financial gain.
Mr Rajab, head of the Bahrain Centre for Human Rights, was released for the second time in late June after being detained during the investigation into this and other allegations. He is likely to serve less than three months because of time spent in detention.
The sentencing of one of the protest movement’s leading voices comes amid a continuing political stalemate in Bahrain, a strategic western ally.
Pro-democracy protesters led by the majority Shia community continue to call for greater popular participation in decision-making as youths clash daily with the security forces of the minority Sunni-led government.
Government critics say the targeting of Mr Rajab, loved by the opposition and hated by loyalists, is a politically motivated punishment of free speech.
The government rejects the charge, saying Mr Rajab has used his social media clout to call for illegal demonstrations, pointing to police injuries sustained by attacks with petrol bombs.
Repealing freedom of expression crimes is one of the key reforms introduced since an independent commission criticised the security forces for their brutality and systematic torture during the crackdown on mass dissent in February 2011 in the midst of the Arab revolts.
Saudi Arabia led Gulf Arab troops onto the island in March to back up a government clearance of the protesters from the now-destroyed Pearl roundabout amid claims by the government that Iran was behind the unrest.
The removal of Sheikh Khalifa bin Salman Al Khalifa, the prime minister of more than 40 years, is one of the protest movement’s main demands.
Observers say the prime minister’s influence has broadened since the crackdown amid strong support from the Sunni community as the symbolic leader of the hardline faction within the ruling family.
Manama says it is implementing reforms of the judiciary and security forces, but the opposition says abuses continue.
The authorities in June announced the discovery of a bomb-making factory in Bahrain, including large amounts of explosives, which some believe could be a sign that hardline opponents may be planning to escalate their tactics.
