Speaking on condition of anonymity, a judicial source told AFP on Wednesday that Rajab was found guilty of spreading false rumors and insulting a neighboring state -- a veiled reference to Saudi Arabia.
The charges leveled against the activist cited laws which bar insulting a neighboring country and insulting national institutions, Reuters reported.
The Bahrain Center for Human Rights and the Bahrain Institute for Rights and Democracy (BIRD), two leading rights groups, confirmed the sentence handed down to Rajab.
The campaigners Twitter account posted a photo showing him smiling and flashing the victory sign after the verdict was announced.
Sayed Ahmed Alwadaei, an activist with BIRD, accused courts of curtailing freedom of expression by deterring Bahraini citizens from criticizing its 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, he added.
Brian Dooley, a senior adviser at the US-based group Human Rights First, called on the US administration to show some spine in standing up to the dictatorship in Bahrain.
It should take punitive action over this verdict and send the American ambassador to visit Rajab in prison to tell him that the US is on his side and against this reckless repression, he said.
Rajab was already serving a two-year sentence over a January 2015 news interview in which he said that the Bahraini regime was torturing political prisoners.
Separately on Wednesday, an appeals courtupheld the death sentences handed to six Bahrainis and stripped two others of their citizenship.
Since February 14, 2011, Bahraini people have been holding peaceful protest rallies on an almost daily basis, demanding that the Al Khalifah family relinquish power and let a just system representing all people be established.
They have also been complaining about widespread discrimination against the Shia majority in the kingdom.
Manama has responded to the protests with lethal force, drawing international criticism.
Bahraini authorities have also detained human rights campaigners, broken up major opposition political parties and revoked the nationality of several activists.
