The Bahrain Center for Human Rights said Rajab was convicted on Wednesday for "disseminating false rumors in time of war", for "offending a foreign country" and for "insulting a statutory body" as codified in the Bahraini criminal code.
Rajab's charges include a social media post on March 26, 2015, the day Saudi Arabia launched an invasion of Yemen. His post criticized wars that "bring hatred, destruction and horrors".
In a statement, Dimitris Christopoulous, president of the International Federation for Human Rights (IFHR), said the "outrageous sentence" on Wednesday "contributes to further shut down" the space for civil society and peaceful protest in Bahrain.
"Nabeel Rajab is not a criminal, he is a prominent human rights defender and political prisoner," Christopoulos said.
He and his organization called on the Bahraini government to "immediately release" Rajab and other human rights activists in detention.
The World Organization Against Torture also weighed in, calling Rajab's trial "a mockery of justice" and his verdict as "surrealistic.
Since June 2016, Rajab has been in "solitary detention" despite his health conditions, according to the Bahrain Center for Human Rights.
The human rights watchdog noted that Rajab's trial had been postponed 20 times, and that the government violated court procedures by failing to give his lawyers enough opportunity to defend their client.
The group also noted that Rajab had been beaten and subjected to "humiliating and degrading" condition while in detention.
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 Bahrainis be established.
Bahrainis have also been complaining against widespread discrimination against the Shia majority in the kingdom.
Manama has responded to the protests with lethal force, drawing international criticism. In March 2011, troops from Saudi Arabia and the United Arab Emirates were also deployed to assist Bahrain in its crackdown.