Alaa Brinji, who has reportedly already served two years behind bars, was found guilty yesterday of a list of charges including "insulting the rulers (and) inciting public opinion," the rights group said in a statement.
Brinji was also convicted of "accusing security officers of killing protesters" in the Shiite town of Awamiya in Saudi's restive Eastern Province, it added.
The group said the journalist had been tried in a "notorious" counter-terrorism court that also found Brinji guilty of "ridiculing Islamic religious figures".
Amnesty's regional deputy head James Lynch slammed Brinji's sentence as "utterly shameful".
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"He is the latest victim of Saudi Arabia's ruthless crackdown on peaceful dissent, where the aim appears to be to completely wipe out any and all voices of criticism," he said.
"Putting someone behind bars for peacefully exercising his legitimate right to freedom of expression, and defending the rights of others to do so, is a complete distortion of the very notion of justice.
Amnesty said Brinji worked for Saudi newspapers Al-Bilad, Okaz and Al-Sharq.
Awamiya, a town of about 30,000 in Eastern Province, has been the scene of repeated incidents since 2011.