Rajab was arrested multiple times in recent years over protests that the government said were unauthorised
Renowned human rights activist Nabeel Rajab was sentenced to two years in jail after a Bahraini court found him guilty of spreading "fake news".
The source, quoted by the official BNA news agency, said Rajab was convicted "of disseminating false news, statements and rumours about the internal situation of the kingdom that would undermine its prestige and status".
The sentence was given to Rajab by the lower court in Manama after he spoke to a journalist about the human rights abuses perpetrated by the Bahraini state.
The verdict, which the source said can be appealed, was immediately condemned by Bahraini and international human rights watchdogs.
Rajab, 52, was sentenced in absentia as he has been hospitalised since April, said the Bahrain Institute for Rights and Democracy.
It denounced the verdict, saying Rajab had been sentenced "for speaking to journalists" and that he had "spent over a year in pre-trial detention, largely in solitary confinement".
"His lawyers state he was denied basic fair trial guarantees," the group said in a statement.
The judicial source said, however, that "all legal guarantees" were provided to Rajab and that his lawyer was present for the proceedings.
Sayed Ahmed Alwadaei of the Bahrain Institute strongly condemned the ruling.
"This outrageous sentence against someone speaking the truth exhibits the brutality of the Bahraini government and its heinous crimes and that of its kangaroo court," he said.
Amnesty International also denounced the decision, saying it exposes a "relentless campaign" by authorities in Bahrain "to wipe out dissent".
"Imprisoning Nabeel Rajab simply for sharing his opinion is a flagrant violation of human rights, and an alarming sign that the Bahraini authorities will go to any length to silence criticism," said Amnesty's secretary-general Salil Shetty.
"Bahrain’s government and judiciary have once again tightened their chokehold on freedom of expression and branded him a criminal," Shetty was quoted as saying in a statement.
Rajab had been arrested multiple times in recent years over protests that the government said were unauthorised.
The Shia activist was pardoned for health reasons in 2015 before being rearrested in June 2016.
He also faces trial for a series of tweets criticising a Saudi-led military campaign in Yemen, with a hearing due on 7 August, according to the Bahrain Institute.
It said he faces up to 15 years if convicted in that trial.
Sunni-ruled Bahrain has been the scene of frequent protests and clashes with police since security forces quelled Shia-led nationwide protests that called for political reforms in 2011.